Implement dns verifyFingerprint.

Implement a new method verifyFingerprint in the DNS plugin that checks
if a given certificate fingerprint is acceptable for the given domain.
This implements already 'sanitising' of the fingerprints as in stripping
colons, spaces and ignoring case.  Thus, applications like Convergence
don't have to implement that themselves.

When this is accepted, I will also supply a patch for Convergence to use the new method instead of sanitising the fingerprint itself.  When Convergence or another application switches to the getTlsFingerprint call, I can implement verifyTlsFingerprint as well (should be trivial when shared code is moved to a function).