Customizable browser bundle

This adds the ability to customize the submodules included in the browser bundle. 

To generate a customized bitcore bundle, you can specify to browserify.js which submodules you want to include, with the -s option:

```
node browser/browserify.js -s Transaction,Address
```
This will generate a `browser/bundle.js` containing only the Transaction and Address class, with all their dependencies. Use this option if you are not using the whole bitcore library, to optimize the bundle size, script loading time, and general resource usage.