Allow single carriage returns as newlines

Apparently there are [still files](https://github.com/atom/atom/issues/1782) that use `\r` for [newlines](http://en.wikipedia.org/wiki/Newline#Representations).

This modifies TextBuffer to accept `\n` `\r` or `\r\n` as newlines.

@nathansobo there are still a few places where we assume a newline contains `\n` and I'm going to add specs to make those fail. Let me know if I missed anything else.

* [x] https://github.com/atom/text-buffer/blob/master/src/text-buffer.coffee#L234
* [x] https://github.com/atom/text-buffer/blob/master/src/range.coffee#L56 

Closes atom/atom#1782