Initial support for alternative level backends

Adds first draft support to use alternative backends via "levelalt", an adapter that wraps leveldb and includes level-js as a db option. 

To use, <code>INDEX=index-level.js npm run build</code> to build a distribution over pouchdb-nightly.js and <code>INDEX=index-level.js npm run dev</code> to test in browser. 
Note: <code>npm run build</code> and <code>npm run dev</code> build and test the standard distribution.