added HTTP namespacing for whitelisted clients

[Pomax]

fixes #224 by updating buildURL and getUploadRoot with a client argument, so we can adaptively set the upload root with or without an /HTTP/ prefix depending on whether the client matches a whitelist for http publication.

[gideonthomas]

r+ with Travis

[gideonthomas]

@Pomax would you be able to update this:
https://github.com/mozilla/publish.webmaker.org/blob/master/package.json#L58
to 1.0.0 or something so that travis passes?

[Pomax]

done and done.

[gideonthomas]

r+