Some usability improvements

[jcpetruzza]

Some changes that have to do with installation, error reporting, configuration and robustness. The main new features are:

• Adds plugin settings, accessible from Preferences -> Package Settings. Changes to settings are dynamically reflected on the plugin.
• Logging to the console is controlled by a verbosity setting.
• An add_to_PATH setting allows the user to give the location of the stack and stack-ide. This is particularly useful on OS X, since programs lauched from the dock, spotlight, etc, get the system PATH but not whatever the user sets via ~/.profile (before this, I could only start Sublime from a terminal with subl)
• When stack is not found on the path, a pop up informs the user of the situation, suggests setting add_to_PATH, etc. (it is not shown again unless the user changes the settings).
• SendStackIdeRequestCommand.__init__() is no longer used to launch a stack-ide instance and kick off the process. Instead, we rely on the watchdog to call StackIde.check_windows() periodically. This method initializes new windows and ends the sessions of those windows no longer open. This way we can have the plugin (re-)trying to start the instances when the path changes, etc.

[lukexi]

Super cool, thanks so much Daniel!!