Adding an "About application" page

@Moxie et al.: as a new initiative, I propose this renewed pull request, which has been rebased to master. A link to a changelog has been removed, as suggested by you. I hope, this PR pleases you.

Introducing:
+ version number read from Manifest
+ urls for license and support
+ based on concepts as found in https://github.com/schildbach/bitcoin-wallet
+ theme-aware page (dark/light design)
+ returns correctly from web pages (like source code, support forum) to the About page
+ (C) 2014 Open Whisper Systems
+ obeying code style guidelines