Added side-by-side diff and other enhancements

New Features:
 - Side-by-side diff option added
 - Converted to a setuptools/distutils style project so packages can easily be made and distributed
 - Fixed some minor bugs and improved the code style
 - Improved command line options:
   * Added flag for accurate mode
   * Added flag for producing a side-by-side diff
 - Added entry point to setup.py file so when installed as a package the "diff_html" tool is available
   on the path.