Change Statuscode 400 to 405 if POST-Request is required

As per http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4.6 change the statuscode from 400 to 405 and include `Allow`-header indicating that POST must be used to grant an access token.
Merge pull request #110 from steffkes/develop

Change Statuscode 400 to 405 if POST-Request is required