Added support for ST3; added links to gem homepages and documentation; refactoring

Hi,

I ported my features and refactorings from http://github.com/leonid-shevtsov/GemLinks_SublimeText to you plugin, preserving such great features as being smart about finding Ruby and being able to open folders in ST2.

* Show gem summaries in the gem list
* Add links to open gem’s homepage and documentation pages to the menu
* Extract more information about gems using a customized Ruby script
* Sublime Text 3: Instead of launching Sublime Text as an external executable, use API commands to open gem folder in new window - robust and reliable
* Refactor searching for Ruby executable a bit (but retained informed guessing for RVM and rbenv locations)
* If no folders are open in the current window, use the currently opened file’s location to find Gemfile
* Do not search for Gemfile in parent directories as Bundler does that internally

I tested this in Sublime Text 2 and 3 in OSX.

I've also added a note about fixing PATH issues on OSX - if everyone just had the FixMacPath plugin installed then we wouldn't have to guess the Ruby path anymore.