adds logic discussed in https://github.com/bshaffer/oauth2-server-php/pull/275#issuecomment-30712851, adds tests from @weierophinney
ensures config values are propogated to HttpBasic and ClientCredentials for new 'allow_public_clients' config value
updates comment typo
allows for null client_secret from getClientCredentials function (as discussed in https://github.com/bshaffer/oauth2-server-php/pull/277/files#diff-756f723f587f0504239170eff6eaade8R40)
merges develop changes
Merge pull request #277 from bshaffer/address-257-allow-client_id-in-post-body-for-user-credentials-grant

adds logic discussed in #275