Parse.com integration!

## Introduction

This PR adds support for syncing across multiple devices. This includes other copies of Marknote, and the mobile app ["Phonenote"](https://github.com/octalmage/phonenote). 

## Changes

This adds a settings GUI used for storing your credentials and enabling syncing. 

![](https://cloudup.com/c_TSH-zN3o2+)

If you enable syncing and put in credentials, Markdown will try to log you in. If the login fails it will try to sign you up. If sign up fails (username is taken) it will let you know! This provides a very clean and seamless experience.
Need to set syncing to false if login/signup failed. This way we don't try to sync if syncing is enabled but we aren't authenticated. 

Also need to remember to disable the syncing checkbox.