Implement Content-Type Filtering (0.3.0a)

Twitter returns links to the media attached to any post in the form of
entities which OGRe can now extract. Errors now raise an exception
instead of just returning. Content-type filtering is now implemented as
a constraint instead of a separate parameter. Also, GeoJSON specifies
longitude first while Twitter accepts latitude first. This
inconsistency introduced a (minor) bug that has been repaired. Some
variables have been renamed, too.