Customizable Theme

Closes issue #73

This is almost finished now and ready for review. There are still a few bugs waiting to be fixed in the app-theme-engine library, but otherwise it's good to go. If you find any issues or if you have any other suggestions just comment here and I'll try to update the merge request as soon as possible.

*PS: I tried to split it in as many commits as seemed possible, but there are still some commits that are rather unorganized, so sorry about that!*

See merge request !11

* fixes the account header top padding on all API versions
* improves the `extractMessage( )` method
* update app-theme-engine

This now shows all the information available (author, time, title and
message)

Move things from MergeRequestActivity into MergeRequestDiscussionFragment and the new MergeRequestCommitsFragment.

Revert DateUtils from lowercasing all time stamps since it was getting us lower case months and such. I really just wanted "just now" lowercased, so I think we are all good

This improves theming of EditTexts in general and also the specific
issue with the EmailAutoCompleteTextView (had to extract the methods
into to LoginActivity and use AppCompatAutoCompleteTextView)

Fix issue with EasyCallback which would cause it to return failure and success (I think) and fix infinite loading state on groups

Make retrofit callbacks better by simplifiing them so that all errors are handled in the onAllError block.

Improved overview layout (for issue #120 )

Title says it all:

![screen1](/uploads/097ea764ca4dd808d9c17c8c53c12347/screen1.png)
![screen3](/uploads/f54f3643da4fc57801712e41e09ef835/screen3.png)

See merge request !13