Use integer values when resizing images

Some custom sizes resulted in fractional heights, which created
artifacts around the edges.

See http://ios.forums.wordpress.org/topic/black-line-added-to-top-of-uploaded-photos-with-wordpress-ios-app-on-iphone-4s
Merge remote-tracking branch 'koke/pr/2' into release/3.6.4
Avoid crashing on invalid/missing featured image

Even though wp.getMediaItem is expected to return a string for the
`link` attribute, it might be `false` if the passed ID is not an
existing attachment or can't get an URL.

If the returned value is not a string, treat it as nil to avoid crashing

resolves #13
Added some sanity checks on post settings

When pre-selecting values on pickers we didn't check if the selected row
was valid and it crashed

resolves #14
Potential fix for categories crash

I'm still not sure how we're getting the "Illegal attempt to establish a relationship 'categories' between objects in different contexts" error, but trying this from something I've read in the dev forums.

Hopefully resolves #15
Bump version to 3.6.4
Added connected_to_dotcom and number_of_blogs params to Crashlytics.
Added some super properties to Mixpanel.
Attaching the dot com username to Mixpanel.
Added log statements to help with post editor crashes.
Register push tokens with the new API
Updated crashlytics
Refactor ReachabilityUtils to use blocks

Fixes crash when launching the app while offline
Updated Chinese (Taiwan) translation
Merge branch 'release/3.6.4' into develop
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into develop

Conflicts:
	WordPress/WordPressApi/WordPressComApi.m