Upgrade httpclient to 5.0

The httpcomponents project is working on the 5.0 release. There is an alpha version of httpclient and a beta version of httpcore. The httpmime dependency that Solr currently uses has been folded into the main httpclient.

I've enlisted the help of olegk from httpcomponents and we'll be using github to coordinate.

This work will not be committed officially until the 5.0 version of httpclient is actually released.