Use callback in Mashify middleware.
Update gemspec for Faraday 0.9.0
Fix Proxy test for all current Faraday versions
Fix Mashify test to work with Faraday callback
Only patch Multipart-Post if neccessary, don't hardcode content-type
Merge branch 'dan/hashie' into dan/master
Merge branch 'dan/faraday-0.9' into dan/master
Add a link to ActiveForce on the readme.

Link to ActiveForce, a gem based on a [gist from @ejholmes](https://gist.github.com/ejholmes/5154415) that wrap Restforce in an ActiveRecord like functionality.