Remove +30s to default download timeout

The 30s were addded with the explanation: "Wait an extra thirty seconds
to account for the call create call's [sic] potential overhead." But the
timeout is only applied to http.Client.Timeout on the actual fetching of
the artifact. http.Client.Timeout documentation states:

  // Timeout specifies a time limit for requests made by this
  // Client. The timeout includes connection time, any
  // redirects, and reading the response body.

Create overhead, either prior to or after the actual fetching, is wholly
independent of the Client.Timeout.

This commit also makes this singularity of purpose more explicit by
consistently naming the parameter downloadTimeout (sometimes née
timeout) throughout the code.
Merge pull request #139 from soundcloud/remove-30s-addition

Remove +30s to default download timeout