Add the mime-types gem as dependency.

The gem is used in the http_client and is normally loaded is Rails but neither
Rails nor this gem was required by the gemspec.
Add WebMock as a development dependency.

I would love to add some unit tests which don't need a connection to a
Matterhorn server to run.
Require WebMock in the spec helper.
Add some file fixture spec helpers.
Add xml fixtures for media packages.
Require mime/types in http client.
Allow track to be added with URL instead of path.

The current version of library assumes the first argument to addTrack is
always a file path. The API also allows a URL to be passed.
Ignore log directory.
Only run remote specs when endpoint is configured.

We don't always want to run specs against an actual Matterhorn install
because we might not have one handy or internet connections are down.
Use a log file for spec logging.

Spec output clears up quite a bit when not logging to stdout. Log is set
to append so it's possible to tail the log.
Merge pull request #1 from Manfred/features/media-package-source-url

Add support for addTrack with a remote URL