Add Java binding support to mcsapi. Required 2 changes to c++ api to make easier to use:

• use const std:string& for return types as java binding can't handle these due to java String being immutable.
• Made ColumnStoreDateTime int part constructor use uint32 even for values which are really uint8_t as this avoids the need for ugly (short) casts in java.

Neither have impact on test cases or python bindings.

See pull request: https://github.com/mariadb-corporation/mariadb-columnstore-api/pull/12

Also see Readme.md for details, obviously you will need a JDK installed, i tested with 8 on centos7. The build uses gradle for the java side which also assists in pulling down junit and mariadb connector jar deps for the tests. This uses the gradle wrapper checked in which will automatically install gradle dependencies if not already present on the box.

Moved to test but keeping me as assignee as I'm probably best to QA it.