Adding Keyword classes for handling of TAL keywords.

Classes were added for 'default' and 'nothing', as well as an
interface, which extends Countable.  Countable is used
to determine thruthiness by other methods
Adding tests for Keyword classes, JSON update

Keyword interface implements JsonSerializable for compatibility
with the json: modifier.

Tests of the keyword classes added
Merge pull request #16 from ajcrites/keyword_classes

Keyword classes