Update configuration logic

[erichmachado]

• Replaces Configuration module with mattr_accessor.
• Starburst.current_user_method now defaults to a symbol value.
• Starburst.user_instance_methods now defaults to an empty array.
• Include private and protected methods when searching for Starburst.current_user_method.
• Adds support for changing the base controller which Starburst::AnnouncementsController inherits from.
• Adds missing specs.

Closes #57