fetching multiple documents from a single request

[fccoelho]

The overhead of making a request for each document one wants to download from a large corpus is too large to be acceptable. A batch download should be available which requires a single request. The set of documents could be returned as a tar-ball.

This issue supersedes #29

[flavioamieiro]

I migrated this issue to NAMD/pypln.web#135 because it implies a change in pypln.web and it's API. As in #40, I'll leave this issue open, but we should probably change it's title and description to address the implementation in pypln.api in case we do it in pypln.web.