Improved to/from dict (load/dump) capabilities

Added Dictable class, which implements _todict() and _fromdict(), and made Abstract Base inherit from it - enables any class in the Coverage Model to be turned into a composable type (dict) using _todict().  The resulting dict can then be brought back to the object form using the _fromdict() classmethod in the appropriate class.
Moved DOA and utility functions

Moved DomainOfApplication and utility functions (is_valid_constraint and get_valid_DomainOfApplication) into basic_types module.  Removed leading underscore from utility functions.

Commented unreachable AbstractCoverage.copy() code
Expanded Parameter Types

Addition & initial implementation of FunctionType

Initial implementation of ArrayType

Improved equality functionality by moving "instance check" into AbstractParameterType.  Subclasses override __eq__ as necessary, calling the super to check instance equality.

Added attributes indicating the _value_module and _value_class to be used for a given ParameterType
Addition of numexpr_utils module

Addition of module containing utility functions for working with the numexpr package (http://code.google.com/p/numexpr/)

Includes functions (see code documentation for usage examples):
* make_range_expr --> enables creation of numexpr expressions that can be understood and evaluated by the FunctionValue class; supports constant and bounded expressions
* nest_wheres --> enables creation of complex (nested) bounded expressions

Added numexpr==2.0.1 to setup.py
Addition of parameter_values module

Initial commit of parameter_values module.  Class hierarchy mirrors the ParameterType class hierarchy.

Includes utility function 'get_value_class' which utilizes the ParameterType._value_module and ParameterType._value_class attributes to return an instance of the appropriate ParameterValue class

Class hierarchy:
* AbstractParameterValue
** AbstractSimplexParameterValue
*** NumericValue (w/initial implementation)
** AbstractComplexParameterValue
*** FunctionValue (w/initial implementation)
*** ArrayValue (w/initial implementation)
*** VectorValue
*** RecordValue
Fixed some issues with ParameterContext

ParameterContext needed some improvements/modifications to enable a smoother interaction with the Dictable capabilities.  Also improved the overall usage somewhat.
Incorp ParamVals; WIP SpatialDomain relaxation

Incorporation of ParameterValue classes.

In SimplexCoverage.insert_timesteps - moved responsibility for expanding the value containers of each parameter to the ParameterValue for the parameter

Made range_values and range_dictionary private-ish by renaming to _range_values and _range_dictionary

Improved mechanism for handling the parameter_dictionary passed in during construction

Added RangeValues class - a simple container for ParameterValues within a coverage

Added DomainSet class - a simple container for the domain(s) that is assigned to each ParameterContext when it is added to the coverage

WIP - Started relaxing the requirement for a SpatialDomain
Updated examples

Updated to reflect changes to the last handful of commits (back to 1f1b31e87a8ada8ca63c4b6cd94f2b6704f178f7)
Bumped ion-defs; removed one-off dumping
Centralized dump/load; Copy dicts entering _fromdict
ConstantType & ConstantValue; cleanup of typing