modularizing level alt

the goal is to make the level alts requirable by build systems (I tend to just say browserify, but that would also make it work in any build system that supports cjs or amd)

- [x] remove `window.Pouchdb` in favor of `var PouchDB = global.PouchDB || require('pouchdb');` this should have the side effect of letting it work in a web worker (memory version especially)
- [x] make sure we don't pull in the full pouchdb source into the bundle
- [x] extract levelalt into a file that can be shared across bundles
- [ ] publish levelalt, and the 3 adapters based on it to npm
- [ ] update build scripts so above is possible