Bind mounts

This is a proposal to fix #14.

I've included:

* the fix to send `binds` in the `start` method
* a test that checks if a file created in the current directory of the local host is present in the `/mnt` volume in the container
* "start with binds" documentation in the readme
* a makefile with `setup` and `test` targets
* documentation on how to test
* added a content-type header on the `post_json` method