Implement base Validator and FieldValidator classes

The Validator class acts as a foundation for all other query processing
steps that require validation. Since everything is constructed
dynamically based on 'meta' data structures, validation is necessary
for resiliency of those structures over time.
Rename existing parsers package to oldparsers
Add function for determining a field's simple type
Simplify setting default objects to registry

Default objects are now named 'Default'. This prevents needing
downstream systems to be aware of what the default class/instance
is by name, but can now simply assume the 'Default'.
Implement Registry.__contains__ magic method
Add support for Registry.__len__

Also implemented __nonzero__ so it
is not falsy.
Fallback to using the length of the object_list if available
Remove Django-specific clause from license
Update LexiconManager.reorder method

Add reverse argument which corresponds to the list reverse
and wrap method in transaction to prevent partially ordered data.
Change reference to formatter choices

This is a cleaner import syntax and enables other use of the
formatter registry.
Rename OPERATOR_MAP and SIMPLE_TYPE_MAP

To OPERATORS and SIMPLE_TYPES, respectively.
Allow for string or model field for determining the simple type
Move comment from group to sites
Remove queryview, add queryable

QueryViews were misused (or not used at all) and resulted tying
the metadata too much to the client. The queryable flag is a more
clear way to denote whether a concept is appropriate for a query
interface.

Fix #68
Remove property decorator for various DataField methods

These methods include `values_list`, `size`, `values`, `labels`,
`codes`, `choices`, and `coded_choices`.

This API change enables greater extensibility potential for these
methods and future ones. This establishes the API convention for
introducing new methods rather than properties.

The immediate change is the repurpose of the `cached_property`, now
called `cached_method`. This now augments two methods `flush` and
`cached` for flushing the cache and checking if the data is cached,
respectively.
Improve subcommands

Introduce DataFieldCommand