Since #89 warrants a semver-major release, I thought I'd cram some more breaking changes that I'd like to do at the same time. So far the PR:

• ⚠️ switches the API from rbush(...) to new RBush(...), a more idiomatic ES approach — since ES forbids calling classes without new, and we still want to expose an extendable class rather than a factory.
• Picking up from Make rbush an ES6 library #89, converts all the code to ES. The UMD build will still be transpiled to ES5 with Buble.
• Removes the format option. While convenient, it still doesn't feel good to rely on eval while developers increasingly adopt strict CSP rules. If we remove it, we'll just document dealing with custom formats by redefining toBBox, compareMinX and compareMinY methods.

To do:

• Remove the format option?
• Add Buble transpilation.
• Clean up and merge search accepts a predicate argument #74
• Update readme / docs.

cc @rowanwins @tschaub thoughts?