Pass custom User Agent to get around WP Engine's strict caching.

WP Engine appears to have very strict User Agent-based caching rules.
It seems that when caching is triggered, all URL parameters are
stripped. Thus WP101Plugin API requests just get served a cached view
of the wp101plugin.com web site. Passing in a User Agent of
"WP101Plugin" seems to get around this.

fixes #11
Merge pull request #15 from markjaquith/issue/multisite-key-invalid

Pass custom User Agent to get around WP Engine's strict caching.