Tidy up Deals class and methods

Add statistics & deals methods to Pipeline.  Add deals method to Stage.
Add deals method to Stage.  Fix camelCase resource_path capitalisation
as required by Pipedrive.  Allow options to be passed to Base.all
method.
Add OrganizationField & PersonField classes
Merge pull request #8 from tjpd/master

Tidy up Deals class and methods