Updated URL.
Fixed README.
Use error object for exceptions

When throwing an exception just by string, then `stack` is not available.
Merge pull request #54 from tenbits/patch-1

Use error object for exceptions
Use `throw new Error` everywhere.
No underscore in main library
No underscore in nlp
No underscore dependency in package.json
No underscore dependency in bower.json
No underscore in tests
No underscore in demo
No underscore in README
Drop polyfills