Support for freezing software like py2exe, pyinstaller or Cx_Freeze.

jsonschema doesn't work when "frozen", it will try to open the draft3.json and draft4.json files using invalid paths.

For instance, in the case of Cx_Freeze on Windows it will try to open something like: C:\path_to_build_dir\library.zip\schemas\draft3.json, obviously this fails because library.zip is a zip file, not a directory!

With this correction, the path would be C:\path_to_build_dir\schemas\draft*.json only when the application using jsonshema is frozen.