Large refactoring/rewrite

Here are the changes:

- add MIT License (kept the original copyright notices)
- bump API version to 27.0
- separate HTTP and CSV handling
- correctly handle Array values as multi picklists (see #10)
- add testing and code metrics ([travis-ci](https://travis-ci.org/propertybase/salesforce_bulk), [codeclimate](codeclimate.com/github/propertybase/salesforce_bulk), [coveralls](https://coveralls.io/r/propertybase/salesforce_bulk))
- rename create to insert (to make it consistent with the salesforce bulk API naming)
- refactor behaviour of wait flag

Still WIP, let me know what you think