Replaced macro for a function per comment on #5.
[#4] Initial zipper implementation. Can be improved.
[#4] Removed unused parent_nodes key from zipper.
[#4] Fixed error in README. Improved IsBranch fun.
[#3] Removed test case.
[#4] Improved zipper representation and implementation.
[#4] Added coverspec.
[#4] 100% coverage.
Merge pull request #7 from inaka/jfacorro.4.implement.zipper

[#4] Implement zipper