v0.6.0

Features:

• Issue unique access tokens
• Define what grants a client is allowed to use

Improvements:

• Corrected class references in doc strings (Josh Johnston)
• Proper handling of errors raised by store adapters

Bugfixes:

• Added missing scopes parameter in SiteAdapter base class (Josh Johnston)
• Deleting authorization token after usage (Josh Johnston)
• Scope parameter not returned by access token response of Authorization Code Grant
• Added missing cache control headers to responses containing a token
• Fixed ClientCredentialsGrant returning a value of 0 of 'expires_in' with refresh token disabled