Replace HTTPie with curl in README, link to API doc

* `curl` is widely installed, and so writing the API examples with
  `curl` removes the HTTPie dependency.
* Let's link to the new REST.md to show off the API documentation.
* Mark the V1 API as deprecated to start moving users to the V2 API.
Reduce header visibility, too much bold
Reduce more bold text, focus on API examples
Merge pull request #130 from mesosphere/readme-v2-example

Replace HTTPie with curl in README, link to API doc