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.
Merge pull request #13 from domob1812/verify

Implement dns verifyFingerprint.