Created Yang to Java Builder pattern class generator

- added Xtend tool to dependencies
- created builder xtend class template
- created BuilderClassDescriptor class which is used as source of information in template
- integrated generating of Builder pattern classes for interfaces
- updated jUnit test

Maven generates .java file as a translation of .xtend template into src/main/xtend-gen. Therefore /binding-java-api-generator/src/main/xtend-gen was added into .gitignore file.
If you use Xtend plugin for Eclipse, you should check a configuration of Xtend compiler in Eclipse, whether there is choosen xtend-gen as an output direcotory.

Change-Id: I72e733c7d8f2587848b0e0d32368f7f1700a44d0
Signed-off-by: Martin Sunal <msunal@cisco.com>