Better localisation

This PR removes all non-stream text domains!

Two things to note:

1. Where we are referring specifically to something which is occurring in another plugin, I'm using `_x` with that plugin's slug for context
2. I have added any additional context where we are referring to WordPress core, because that's the default context, anyway. For example, `_x( 'Posts', 'default', 'stream' )` is not very helpful.

@themeboy @deckerweb - I'd love to get your thoughts on this approach.

@fjarrett please review. It would be great to release this with 2.0.1.