Fixes #737

Use string builder over looped string concatenation.

ABOUT.md: remove confusing sentence

ABOUT.md: update formatting for Nextercism

[WIP] Adds additional test for two-fer exercise

Choose core exercises

house: add to track

Update config.json to set which exercises are core and which ones are
unlocked by other exercises.

Fix broken link - exercism.io: Installing Java

Series example refactoring

I fixed the link path to end in "installation" instead of "installing"

Use Collections.singletonList in custom-set tests

Fix spelling errors in custom-set, word-count test names

Update gitter webhook for travis integration

Add default maintainers file

We wish to feature the maintainers of each track on the new (nextercism) website. Ideally, the bios will be tailored to each track, giving a friendly view into each maintainer's background and interest in the language, and even (perhaps) what they enjoy most about maintaining the track, or what type of things they tend to focus on.

The main reasons for including these are to:

* provide a stronger basis for empathy with maintainers
* give well-deserved recognition
* plant a seed in people's minds that _Exercism tracks are maintained by people like me... I could do that!_

Being featured on the Exercism website is entirely optional, and we're generating all the data with show_on_website=false so that if you don't want to bother, you don't have to change anything.

If you've moved on, or decide to do so, please submit and merge a PR (for visibility to the others involved) that sets **alumnus** to _true_ for your maintainer entry. I'm working on a bot that will update the GitHub team to reflect the current state of this file.

We can merge this as is, and any changes can be added in subsequent pull requests.

update TUTORIAL.md [Hello World!] exercise

sum-of-multiples: replace forloop

in this first exercise, i face the problem that Gradle was not installed on my device. the tutorial doesn't mention this part. even before i fetch the exercise there was no mention to the Gradle in the installation. so I think as a beginner it might help to add this part to the new people will try and use the tutorial in future and some of them will probably face this issue.

TESTS.md: fix link to tutorial

nucleotide-count: rename classes

Remove redundant key/value pairs from config.json

The slug is not used anywhere. We initialize a track based on knowing the Track ID.
Since the repository is always named after the track ID, this field, too, is redundant,
as it can be inferred.

forth: add to track

rotational-cipher: estimate and tidy

Replace exercise templates with track-level default

You can still override the track-level default template by adding a .meta/readme.go.tmpl for the exercise in question.

This removes the stray newline at the end of the generated READMEs.

Update links to reflect changes in the docs repository

twelve-days: default sing method