Don't bind http properties which play uses.

TODO - We should convince them to use a play namespace.  Right
now we assume the http.*, https.*, sbt.* and ivy.* namespaces are the
correct ones to pass through.  We should:

(a) Make this configureable
(b) Try to figure out who isn't namespacing the environemnt vars.
Merge pull request #23 from sbt/wip/fix-play-port-bind-issue

Don't bind http properties which play uses.