The extension was not properly build with the “python setup.py install” command, it worked fine with “python setup.py build” but the building wasn’t executed when invoking “install” directly.

When rules are passed as strings, the line number would not be shown
when input as a string. The information is available however.

If it is not a file like object, the function would return -1. This
would eventually lead to a segmention fault later on.

The "scan" method was returning a error when the "externals" dictionary has keys that weren't previously passed to "compile". Ignore those extra keys to avoid regression errors.

Show appropriate error message when an external variable is redefined with a different type. See issue #12.