Prompt when switching blogs if a new post already has changes.

Preserves the changes we can.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/951-prompt-changing-blogs

Conflicts:
	WordPress/Classes/EditPostViewController.m
Merge pull request #974 from wordpress-mobile/issues/951-prompt-changing-blogs

Prompt when switching blogs if a new post already has changes.