Feature filtering

This addresses #1299, #1871, #2033 

Adds:
* flexible feature definition
* sidebar to allow user to toggle features (hotkey 'F')
* info bar on bottom to warn user when features are hidden
* supports auto-hiding features (currently auto-hides >100 points, >100 buildings)

Not ready to merge yet, but looking for feedback.
I really like how this is coming together - dense areas are way easier to edit now.

TODO:
- [ ] tests
- [ ] replace hamburger icon
- [X] adjust text/tooltip language?
- [X] better handling of multipolygons? (e.g. should really hide inner 'holes' when hiding an 'outer' landuse)
- [X] restrict some edits if a feature is hidden? (e.g. moves, deletes?)
- [X] add wireframe mode? (#813)
- [X] make gpx tracks features instead of background? (related #2352, #2335)

![screenshot 2014-09-16 16 02 48](https://cloud.githubusercontent.com/assets/38784/4294306/5bbdca92-3dde-11e4-9faa-339e99e47acc.png)

I believe prepending a '/' makes it an absolute path, so it only works if iD at the root path. It would fail on http://www.openstreetmap.org/edit for example.