Kill code

This should look familiar -- it's a simplified version of my old "shrink" pull request (https://github.com/danvk/dygraphs/pull/263).

This pull request does the following:
- Simplifies & standardizes a few methods (warn/error, findPos).
- Removes the weak dependencies on RGBColor and strftime libraries.
- Removes stacktrace.js from the minified library.
- Moves custom circles into extras.

This shrinks the minified code size from 141427 --> 126509 bytes (10%). It will also simplify some later closure changes.

What this doesn't include from the older pull request is the mass removal of exported symbols. That's better handled by an "exports" file in closure-land.