Full closure

Here's my attempt to fully closurize dygraphs.

Highlights:
- Comiles with no errors or warnings.
- "make test" all tests pass
- "make test-combined" about half the tests pass
- This resulted in a "dygraphs-exports.js" file which should be helpful for other projects.
- Minified code shrinks by ~40% (140165 -> 85777 bytes), gzipped code by ~25%.

I'm still working on getting the tests to pass with dygraph-combined.js, but some high level comments would be welcome at this point.