Shrink the size of dygraph-combined.js

Stats:
- 135782 -> 109897 = 19% reduction
- 40827 -> 33214 gz = 18.6% reduction

Techniques:
- Removed dependencies on strftime, rgbcolor and stracktrace libraries.
- Moved most of Dygraph.CIRCLES into extras/
- Switched to a better JS minifier (uglifyjs)
- Used the "module pattern" to replace private static methods with local vars, which can be mangled.