Fixes incorrect exception type for ops while closed

Replaces ValueError with IOError when operations on a closed coverage are attempted.  Was already IOError in some spots…this corrects the 2 cases where it was ValueError

Updates tests appropriately
Adds support for "pv_callback" arg

Provides support for a specialized argument "pv_callback" within ParameterFunctions.  When this argument is encountered, the 'value' is assigned to the parameter value callback with the appropriate slice:

	if arg == 'pv_callback':
		arg = lambda arg: pval_callback(arg, slice_)

Adds test to verify functionality
Merge pull request #102 from blazetopher/super_callback

Support for "pv_callback" arg in ParameterFunctions