Better localisation

[lukecarbis]

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 not 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.

[fjarrett]

Nice work.