Adding Buzz package to composer.json

Current package relies on buzz, which is not installed when using composer.  This can be remedied by including "kriswallsmith/buzz": ">=v0.9" in you higher-level composer.json, but should not be necessary as buss is a dependency of mopa/wsse-authentication-bundle
Merge pull request #6 from westinpigott/patch-1

Adding Buzz package to composer.json