Improves coverage load mechanism; minor fixes

Improves the .load function for coverages such that *Coverage.load will load the correct coverage based on the .coverage_type attribute regardless of the abstract/concrete used (i.e. AbstractCoverage.load & SimplexCoverage.load will result in the same type of coverage - even if it's a ViewCoverage!!)

Exposes AbstractCoverage in coverage_model/__init__.py

Provides full comparison functionality (via functools.total_ordering) for Span objects
Merge pull request #60 from blazetopher/improve_load

Improves coverage load mechanism; minor fixes
Adds new files supporting the refactored nosetests framework.
Adds time axis to 'time' parameter.
Continues migration from test_coverage_basics.
Continues migration from test_coverage_basics.

Replaces uppercase parameter names to lowercase in tests.
Comments out migrated tests in test_coverage_basics.
Removes working directory if exists.
Completes migration from test_coverage_basics.py

test_slice_and_dice omitted from tests due to suspected duplication with test_get_by_slice.
Adds get_props decorator function

Adds get_props() decorator function and TESTING_PROPERTIES class dictionary to enable subclasses to provide properties on a class-wide and test-by-test basis.  This allows the tests defined in CoverageIntTestBase to remain generic and flexible.
Adds tests and minor cleanup.

Adds test for checking the list_parameters() function.
Refactors assertions using SimplexCoverage to AbstractCoverage.
Adds override for empty coverage test.

Added override for test_get_by_slice.
Adds coverage for _axis_arg_to_params.
Moves CRS tests to coverage_test_base.py
Adds ViewCoverage tests.
Deletes unused class.

Class migrated to coverage_test_base.
Adds test_crs_values.

Migrates standalone CRS test functions to new class.
Migrates standalone parameter tests.

Moves standalone parameter tests to test_parameter_values.py so they do not run more than once.
Updates function names and usage.

Renames functions to be in-line with test performed.
Removes usage of num_timesteps and OMIT_TEST_NO_DATA to test for an empty coverage.