Add Piximedia adapter

[christopher-allene-piximedia]

Hello Team,

We would like to add our Piximedia adapter to Prebid.

This PR also include full test coverage for our adapter (100 % statements, branches, functions, lines).

Here is a sample bidder configuration for testing everything works as expected:

{
    bidder: 'piximedia',
    params: {
        siteId: 'PIXIMEDIA',
        placementId: 'PREBID'
    }
}

We have set-up a test example showing how everything works together:
http://resources.pm/sandbox/prebid/

Feel free to contact me if you need more info, or check out our web site: http://www.piximedia.com/

Thanks!

[matthewlane]

Getting bids back and all tests pass. Thanks for the adapter and the extensive test coverage!