Refactor options processing

* Create Ghn::Options class to manage options
* Remove options processing code from bin/ghn
Refactor command processing

* Create Ghn::Command class to manage command
Refactor access token processing

* Create Ghn::Token class to manage access token
Refactor Ghn initialize and arguments

* Change initialize arguments of Ghn
* Add accessor of Ghn::Command (command, args)
More refactor

* Enclose run and print (open browser) logic into Ghn.run
Fix test
Bump version to v0.0.2
Merge pull request #8 from kyanny/refactor

1st Refactor: separate classes by its responsibility