Don't publish blank posts.

Fixes #1023.

- Not triggering publish if the post is new and has a blank excerpt, content and title
- Fetching of new post data and actually saving the post separated.
Merge branch 'issue/1023-dont-publish-blank-posts' of https://github.com/rase-/WordPress-Android into rase--issue/1023-dont-publish-blank-posts
Show Toast to the user when empty post published.

If the post is empty (and can't be published), the user is shown a Toast
with the text "Can't publish an empty post".
Merge branch 'issue/1023-dont-publish-blank-posts' of https://github.com/rase-/WordPress-Android into rase--issue/1023-dont-publish-blank-posts
replace Toast instanciation by ToastUtils.showToast()
Merge branch 'rase--issue/1023-dont-publish-blank-posts' into develop