Several changes improving input consistency across devices and zoom levels

- Added function to View for transforming distances from screen space to world space
- Use displacement instead of velocity to actuate panning input (reduces integration errors)
- Clarify expected coordinate systems for input functions
Fix typos
Brevity
Too many flippin signs!
Merge pull request #32 from tangrams/input-scaling

Improve input consistency across devices and zoom levels
Merge branch 'master' of github.com:tangrams/tangram-es into lights
Mergin with master