New features + refactor

This branch implements a few features:
 * Open note: fetch from Evernote, convert to markdown
 * Update note: save edits to an opened note back to Evernote
 * Caching of notebooks
 * Markdown2 updated to 2.2.1

Also, I refactored the code a bit so that commonly used functions are methods of a common base class.

There are few opinable changes but since they follow common ST practice they should be fine:
 * Context menu entries are moved either to commands in command panel or in the Main menu
 * The name of the plugin does not include the redundant "Sublime" prefix

It would be nice if this could be merged with the official SublimeEvernote plugin in package control, after thorough testing in ST2.