create the curl plugin

Summary:
This plugin downloads a single file and puts it by itself in a directory. This
is for e.g. fetching prebuilt JARs or jquery.js. Like the 'cp' plugin isn't
actually backed by `cp`, this isn't actually backed by `curl`. It uses Python's
`urllib`, which supports a similar variety of URL schemes (including `file://`,
which we use for testing). My other preferred name would've been "http", but
since the plugin is more generic than that, we need a more generic name.
Suggestions please.

Test Plan: Fetch and reup tests.

Reviewers: sean

Reviewed By: sean

Differential Revision: https://phabricator.buildinspace.com/D24