Added a "Geohash mode" setting: Current graticule / Nearest.

Also fixed a bug where ListPickers would have the wrong SelectedItem upon initializing, and fixed the MainPage title (it's not really "today's hash" any more...)
Replaced the BouncyCastle excerpt by an own implementation of the Message-Digest 5 Algorithm (MD5), straight off RFC 1321.

GAH, writing this wasn't fun at all. Debugging two threads in parallel just feels buggy. But now it works, and looks a lot cleaner than the BouncyCastle version (no unneeded inheritance etc.). It passes the standard MD5 test suite.
Fixed several Code Analysis warnings