Rename MainApplication to App

It makes sense to have a short name because it's used in so many places.
The name "MainApplication" isn't any more descriptive anyway.
Merge branch 'rename-mainapplication-to-app' into offline-guides

Conflicts:
	App/src/com/dozuki/ifixit/model/Image.java
	App/src/com/dozuki/ifixit/model/search/GuideSearchResult.java
	App/src/com/dozuki/ifixit/model/search/TopicSearchResult.java
	App/src/com/dozuki/ifixit/ui/BaseActivity.java
	App/src/com/dozuki/ifixit/ui/GuideItemView.java
	App/src/com/dozuki/ifixit/ui/auth/LoginFragment.java
	App/src/com/dozuki/ifixit/ui/auth/RegisterFragment.java
	App/src/com/dozuki/ifixit/ui/gallery/MediaFragment.java
	App/src/com/dozuki/ifixit/ui/guide/StepEmbedFragment.java
	App/src/com/dozuki/ifixit/ui/guide/StepVideoFragment.java
	App/src/com/dozuki/ifixit/ui/guide/ThumbnailView.java
	App/src/com/dozuki/ifixit/ui/guide/create/GuideListItem.java
	App/src/com/dozuki/ifixit/ui/guide/create/StepListItem.java
	App/src/com/dozuki/ifixit/ui/guide/create/StepReorderFragment.java
	App/src/com/dozuki/ifixit/ui/guide/view/FullImageViewActivity.java
	App/src/com/dozuki/ifixit/ui/guide/view/GuideViewActivity.java
	App/src/com/dozuki/ifixit/util/Utils.java
	App/src/com/dozuki/ifixit/util/api/Api.java