Merge pull request #17 from lukecampbell/params

Updated and expanded ParameterTypes
Revert "Removes dirty checking from *Manager classes"

This reverts commit 70ba8ca6572c6fa7e574c08f0b597d3e490d137e.

Only reverts portions relevant to dirty checking.  Changes to coverage.py and test_coverage_basics left intact.
Changes to utils.hash_any over internal ._dohash()

Switches to utility function - identical to ._dohash
Merge pull request #18 from blazetopher/fixes_cpu

Fixes cpu usage
Adds pythread support with gevent sync.
Comments and removed extra func
Merge pull request #19 from lukecampbell/thread_manager

Adds pythread support with gevent sync.
Adds cleanup, context mgmt and patches test.
Merge pull request #20 from lukecampbell/thread_manager

Adds cleanup, context mgmt and patches test.
Migrates to new location for hash_any

Also updates pyon and ion-definitions to latest
Increases timeouts in test
Merge pull request #21 from blazetopher/hash_move

Hash move
working poc for numexpr
Adds support for Dictable types in lists/tuples
Adds and integrates *Expression classes

Adds AbstractExpression, NumexprExpression, and PythonExpression classes in coverage_model.parameter_expressions module
Integrates the *Expression classes into the ParameterFunctionType/Value classes.
Adds example "sbe37im_samplecov" showcasing usage
Adds coverage_model.utils.express_slice

utils.express_slice replaces any None components of a slice_ with real values

For example - given a total domain of (20,10):
(slice(None,None,10),slice(3,None,None)) ==> (slice(0,20,10),slice(3,10,1))
Adds value caching to SimplexCoverage

Adds capability to cache the returns of calls to get_parameter_value to improve repeated call performance
Adds memoized_values to PFV
Allows retrieval of ind params & fun maps

Adds get_function_map and get_independent_parameters functions to ParameterFunctionType.

get_function_map returns a nested dictionary for all *Expressions required by the ParameterFunctionType's expression

get_independent_parameters returns a list of the independent parameters required by the ParameterFunctionType's expression
Adds pydot dependency; get/write_dependency_graph

Adds functions (get_dependency_graph and write_dependency_graph) for retrieving the graphviz DOT style digraph for the ParameterFunctionType's *Expression