Fixes #425

This PR adds the following:

• Implements XOFF (ctrl+S) and XON (ctrl+Q)
• Separates writes from the processing of the writes by means of a write buffer (Terminal.writeBuffer) and setTimeout(...,0) to start the write async
• XOFF is sent to the pty process when a threshold is reached so that xterm.js can catch up
• Writes are batched in separate async calls in order to prevent the possibility of overloading the UI thread. Calling them all async is bad for performance as it means that the renderer would need to catch up before every write.
• Frames are now skipped in order to save CPU time for the processing of data, not useless rendering of output that will be replaces in ~1000/60ms

Notes:

• Since things are not synced with the backing process programs such as time are now accurate!
• ^C is now responsive
• There is a large diff due to an indentation change in write (now innerWrite), you only need to look at the beginning and end of the function though