Bug fix and minor enhancements.
- Bug fix: Fix a memory leak in pacparser_find_proxy(). (Issue #22)
- Enhancement: Use PACPARSER_DEBUG environment variable rather than just 'DEBUG' to specify verbose logging. (Issue #23)
- Enhancement: Improve error handling. Allow applications to control error reporting. (Issue #24)
- Enhancement: Make pacparser python module compile on Python 3 as well. (Issue #21)