[refactor] Reorganized Gemfile
[refactor] Allowed adapter parameters to be variable; specific adapters can define the parameters they require
[refactor] Moved adapters to Houston::Adapters, allowed configuring more than one adapter in config.rb
[refactor] Replaced "Goldmine" with the generalized concept of "Antecedents" which is defined by config.rb
[refactor] Omitted the actual module from Adapters.each's parameters (unneeded)
[feature] Added a readonly adapter for retrieving tickets from Github Issues
[skip] Cleaned up some junk in the code