Double click or enter key to navigate results

This addresses https://github.com/ameyp/CscopeSublime/issues/7 

It changes the CscopeVisiter class inheritance from sublime_plugin.EventListener to sublime_plugin.TextCommand. This allows us to define key and mouse events in *-mousemap and *-keymap files. I've tested this in OS X, but I believe it should work correctly in Linux and Windows too.