Updates to the plugin handling

-Changed the register_pvs function to accept no parameters, but return 2
dictionaries.  (This is what the docs said it should do all along.)
- Updated the doc string describing the plugin handling.
- Added code to allow plugin dirs to be specified in the config file.
- Also, some minor updates to the command line and config file parsing
code
Pass process variable name to calculation functions

Add the PV name to the list of keyword params passed to all the PV
calculation functions.  (Useful for cases where one function may handle
more than one PV.)