Get rid of 512k static size buffer in generateFingerprint()

* also move to guava sha1 invocation variant, which simplifies the code
* fixed typo in test case
* full test coverage
Remove silly test case (I have reevaluated my full test coverage idea)
Merge pull request #8 from spotify/public-keys-dynamic-buffer

Get rid of 512k static size buffer in generateFingerprint()