Merge in Gterm

I've finished the basic Git terminal emulator class, which will handle the 'git X' statements typed in by the user and will allow to compare statements with the required statement in the course. At the very core there are *Gterm::Statement* classes, with *Gterm::Statement::Base* providing options assignment, unaliasing and validation. Best way to compare two statements is using *Gterm::Statement#parse* which returns a *Gterm::Statement::Base* class implementing *#==*. Best way to use it as a virtual terminal is to instantiate *Gterm::Terminal* with a repository path.

Tests are in *spec/libs* - they create a repository in *tmp*, so be sure you have write permissions to that folder while running them. 

I've added YARD for the docs, you can use *yard server* to check out the documentation for the Gterm library.