Add error messages and logging

[gczuczy]

Hello,

Flask uses heavy use of python's logging module, probably would be a good idea to also use it in flask-kerberos.

The _gssapi_authenticate call is completely swallowing diagnostic messages. When something is not working, all we're getting is a 401/403, without any traces or what the hack is going on. The kerberos.GSSError exception contains quite a bit of useful information, which is not being used, but swallowed by a None return. Also, when the nit is failing or anything is happening, there should be logging statements all around the place, with ERROR/WARN levels.

Please improve the library with proper error message propagation. Make it sysadmin friendly.