value.js, JSHint.

- lib/value.js is a pretty close port of value.dart, with the follo
wing changes:

  - ValueType.NUMBER is redundant. All non-integer numbers in JavaS
cript are doubles.
  - Since JavaScript doesn't have types, toInteger/toDouble/toPrimi
tive/etc are pretty useless.
  - Instead of Value.of(), all of that is just baked into the constructor if it is not provided.
- With value.js done, it will allow me to start working on node.js (DSNode, but that's kind of funny).
- Added a small Gulpfile.js with JSHint. It will also manage tests later.