Fluent API

What's done:
- This builds on the work by @olibob and extended by me, but takes it in a different direction.
- I've added extensive RDoc documentation to the classes and added a bit more ceremony to the gemspec and other documentation.
- It works, at least as far as I've tested (using uploads). The best example right now is in bin/cloudconvert. Ideally, this command-line tool would be able to specify anything that can be specified, but I haven't done this.
- This looks like a substantial rewrite, but it isn't really. I have mostly refined interfaces that @olibob and @pandurang90 provided. The main functionality that I wrote here are the process payload builder classes.
- I've made it so that configuration intuits as much as is possible.
- The Process class is now pathologically incapable of being started more than once. This exactly mirrors the behaviour required by CloudConvert. There is no need to call #new_process anymore; the new process ID is created automatically when Process#run or Process#start is called.

What's not done:
- There's still no tests.
- The README has not been updated with current examples.

Thoughts and Notes:
- Even though in my contribution documents I always tell people never to touch the version, I've touched the version.
- I generally follow a modified version of semver in a couple of ways. To me, the 0.x.y version series is fairly free to change the API as long as the minor version clearly indicates that there are possibly breaking changes.
- If this were my project and gem, I'd *probably* make this version a 0.8 or 0.9 release with a fast-track toward a 1.0 release once the tests are in place.
- I'd also do release management with Hoe…but I've got a few gems out there that I release pretty regularly and have built my workflow around Hoe. I might consider using https://github.com/postmodern/rubygems-tasks these days if I were starting fresh. Maybe.

Sorry about the size of the changes here.