Added the ability to filter ActiveSerializer attributes

e.g. for [attr1, :attr2, :attr3

{only: [:attr1, :attr2]} => :attr1, :attr2
{except: [:attr1, :attr2]} => :attr3
{only: [:attr1], except: [:attr2]} => :attr1

'only' takes precedence over 'except'
Altered filtering to handle empty/singular criteria
Refactored filter validation - better method name & return values
Merge pull request #1 from PeterHamilton/active_model_filtering

Added the ability to filter attributes using 'only' and 'except' with ActiveSerializer