Allow passing params while making GET requests to openx API

[arseniy-panfilov]

I've found it somewhat inconvenient that one has to prepare the whole url manually in order to make a Client.get request. This means you should use urllib.urlencode or something like that to construct the query string.

This little fix makes it possible to streamline params argument to requests library which is capable of handling them correctly without any hustle:

client.get('/report/run', {'start_date': start_date, 'end_date': end_date, 'report': 'inv_rev'})