Feature #1047: Chirp support for Python virtualenv's

This change adds support for virtualenv's to Chirp. 

The bulk of the changes involve updating the platform classes to reliably
determine the location of the runtime directories for locales, pixmaps,
xml schemas, and the stock configs. The other changes are updates to use the 
new platform class functions, and to remove some absolute paths.

Technically, these changes aren't virtualenv specific - they just make 
Chirp more 'location aware', so virtualenv's can be used.

Change summary:

chirp/platform.py 

    1. adds five new functions to return runtime directories -  locale_dir(),
       share_dir(), stock_configs_dir(), pixmaps_dir(), schema_dir()

    2. updates the logic to determine the runtime locations of the runtime
       directories.

    3. adds tests for the new functions.

chirpui/mainapp.py

    1. updated to use the new platform functions for the stock_configs_dir
       and the pixmaps_dir.

chirp/generic_xml.py 

    1. updated to use the new platform function for the schema_dir (xml schema files)

chirpw

    1. updated to use the #!/usr/bin/env idiom, so it picks up python from the path 
       instead of a hard coded location.

setup.py 

    1. change the rpttool path to relative instead of absolute.