The binary-search tests force the practitioner to generify the class
but only one type (integer) was actually being used in the tests.
This should be changed so that more than one type is actually used, to
make it more clear that generics are necessary and to demonstrate how
generics is used.

As discussed in #230, move linked-list to one of the last difficulty 6
exercises, so that binary-seach is the first exercise that introduces
generics.

Add a hint to the binary-search exercise to explain generics as this
is now the first exercise to require it.

Fixes #230.

* binary-search: make test class less DRY
* binary-search: use collections emptylist and singletonlist

With exercism/exercism.io@54e1df3, we can now gather exercises into a
subdirectory. By separating the language track files from the
configuration and management files, it is easier to read the project.

- move gradle config under "exercises" too as it would make a mess
  otherwise.

Gradle can be run directly on Mac/Linux/Windows and
auto-download dependencies (much like sbt for Scala),
as well as simplify IDE integration.

Add appropriate .gitignore for this sub-tree.

For developers, to add a new exercism:

    cp -r _template new-exercism
    # create test in new-exercism/src/test/java/SampleTest.java
    git add new-exercism

Users will be able to just run "./gradlew check" to run tests.

To use this template to create a new assignment,
simply cp _template to the name of your new assignment
and drop in the appropriate tests.