Support image mime types

This adds another specific route for image/* mime types
Maybe could be unified a bit with the multipart handling
in that code section. Creating a parser in the user code
depends heavily on the .emit('end') sequence if it works
or not...
Add content type image/png test case
Merge pull request #406 from kof/master

Make Request.prototype.agent chainable
Merge pull request #300 from nebulade/master

Support image mime types
Upgrade cookiejar dependency
Stop testing 0.8.x to keep travis happy
Release 0.18.1