Merge pull request #54 from abernardes/master

Infer undefined schemas from Sequel adapter

closes #39
Update README.md
Simplify benchmark thanks to schema inferring
Make use of headers when building mappers for grouped relations

  * Add Reader#header
  * Add RA::Operation::Group#header
  * Wait with building mapper model until very end

refs #53
Remove unused ivar from mapper builder
Remove auto-generated constant in after hook in spec
Add mongo adapter because why not
Add mongo service to travis