Update dependencies

Fix deprecation warnings
Update elixir dependency requirement to 1.3

Moved OAuth2 errors to specific modules.

Minor bug fix.

Added and applied string utils.

Added blank lines to the EOFs.

v0.6.1

Fix namespace typo for renderer

Refactoring.

Namespace changes.
Authentication & GrantType Behaviours added.
OAuth2 error specs implemented.
OAuth2 errors standardized into tuples.
Authenticate & UnauthorizedOnly plugs added.
Updated readme & documentation.
Added priv_settings map to User and Client models to save non public settings like email_confirmed, otp_secret and etc...