* Add LCOV functionality into coverage.py

* Add testing for the LCOV reporter

* Add documentation for the LCOV reporter

CoverageException is fine as a base class, but not good to use for
raising (and catching sometimes).  Introduce specialized exceptions that
allow third-party tools to integrate better.

Because they are warnings issued while parsing the configuration file, it's not
possible to suppress them with the coverage configuration.

This makes exceptions report their causes correctly, as "The above exception was
the direct cause of the following exception" instead of "During handling of the
above exception, another exception occurred."