Middlewere for getting artifact information

All artifacts from clojars are fetched.  There's no 'get everything'
endpoint for the mvn central api.  This is unfortunate, but most clojure
libraries appear to be on clojars.  One notable exception are all the
libraries found under the groupId org.clojure.  These are explicitly
fetched.

If we ever have to get more libraries from mvn central that code should
probably be re-written with core.async so we don't spin up too many
threads for web requests.

Two new ops are added:

- artifact-list wich returns a list of all available artifacts
- artifact-versions which returns a list of all available versions for a
  given artifact
Update readme

- The readme was quite long, it's now a bit shorter.
- Remove section about selecting individual middlewere, I personally
  found this confusing in cider and there's really no reason not to go
  with the easier plugin option
- Move some general info out of the `usage` section
- Move the `clojure client` section in under usage
Fix some bugs

- Force was always truthy
- The deref of the futures for mvn central was never done, because of
  lazy sequences
Merge pull request #3 from clojure-emacs/artifacts

Middlewere for getting artifact information