pep8 compliance, implementing raw_query

This PR:

* changes public function names to lowercase
* adds a `raw_query()` public function for complete QL queries.
* removes the stub of a `search()` function (better implemented in the costom queries.)
* raises the `OverpassException` rather than exiting with a print statement.
* adds some docstrings.
I don't really like passing `asGeoJSON` around like this... Perhaps better make it a private variable in the API class?