List ractive/backbone as peerDependencies

Fixes #19. Also:

- document usage notes on how to use it with npm/browserify.
- test ractive 0.4.0 support (since we list 0.3.9 as the minimum version).
- change ractive version criteria from `~ 0.3.9` to `>= 0.3.9`.
- change backbone version criteria from `~ 1.1.0` to `>= 1.0.0`. we have test suites to ensure 1.0.0 compatibility.