Merge pull request #353 from sfuhrm/gmail-style-user-pics

Gmail style user pics, 2nd try

* sfuhrm/gmail-style-user-pics:
  Changed the fallback char from 'K' to '?'. The riddler was here ;).
  Using Android proposed colors as contact color palette now: http://developer.android.com/design/style/color.html
  Fixed NPE found by blackbox87 ... thanks pal!
  Added more finer characters as proposed by cketti
  Caching also the calculated anonymous bitmap as proposed by maniac103. This removes a lot of code for special handling unknown contacts.
  Bugfix for negative modulo result indexing the palette array
  Changed hash based color calc to a hash indexed palette as discussed in the pull request.
  GMail-app-style generated colorful one-letter contact pictures for pictureless contacts

Conflicts:
	src/com/fsck/k9/activity/misc/ContactPictureLoader.java
	src/com/fsck/k9/fragment/MessageListFragment.java