db: switch level dependency to levelup + leveldown

This is a first step towards making search-index fully browserifiable. See #20.

Note that currently version of levelup at the time of this writing depends on
leveldown@0.10 (not 1.0).
db: add level.js dependency and a browserify entry point.

search-index has been changed to accept a `db` property in its options, which should
be a valid LevelUP-supported database (like level.js or memdown).

search-index-browser.js is a browserify entry point that creates a level.js db and
passes it in as an option to search-index.
browserify: basic build.

The core browserify build is now working.  Try it with:

node_modules/.bin/beefy test/spec/0indexing-spec.js -- -i WNdb -i lapack

(The two -i options are to ignore unused modules
that are conditionally require()'d in NaturalNode,
but not declared as dependnecies and thus not
installed.)

Still something not quite right with
initializing the database, but getting close!
fix test references to main module
db config: remove restriction on indexPath && db
remove unused require()'s

Browserify (when used along with brfs transform) was failing on
these when they were in a comma-separated `var` declaration.
Just remove them since they're not being used.
browserify: setup testing in browser w/ jasmine.

Tests still failing in the browser, but they're running.
Use:

node_modules/.bin/beefy test/spec/0indexing-spec.js -- -i WNdb -i lapack

And then browse to http://127.0.0.1:9966/test/index.html
browserify: add working browser example

A basic example showing that indexing and searching work in the browser.  Run using beefy:

`node_modules/.bin/beefy examples/basic.js -- -i WNdb -i lapack`

and then browse to http://127.0.0.1:9966/examples/basic.html
Merge pull request #59 from anandthakker/master

make search-index module browserifiable