Cross-browser mousewheel support (including IE). Optimisations.
Optimizations are from profiling on Firefox. They include:
- Reducing calls to getCTM() (viewportCTM is no longer a static variable)
- Cache calls to getScreenCTM() which is slow on Firefox

Made some attempts to normalise wheel speed across browsers.
Tested manually on FF 28, Chrome 33 and IE 11.