Plugin naming tests

[e2]

Properly test the plugin naming in all combinations (and show warnings/deprecations):

• converting guard name to guard module (normal scenario)
• converting guard name to guard gem name (when gem is not required e.g. by Bundler)
• converting guard gem name to module name (just for the sake of RubyGems)
• converting gem name to template path (during guard init)
• converting gem name to guard (for detecting guard presence in guardfile during guard init and guard show)
• converting guard module to guard gem (for future API)
• converting guard module to template path (for future API)
• converting guard module to require path (for future API)
• using module name as guard name (for future API)
• using module as guard name (for future API)
• using project name string as guard name (for future API)

(Best implemented in Guard as e.g. PluginModuleName class representing a symbol containing full namespaced plugin constant - e.g. Guard::Foo::Bar and allowing conversion to-and-from anything else).