Cleanup styles + reformat default display

Reworked a lot of reference entries with better default displays + examples:

If type=uri, show examples:
![screen shot 2014-09-04 at 12 55 26 pm](https://cloud.githubusercontent.com/assets/108094/4153144/5842d3e0-3455-11e4-87ee-654e8b76b62a.png)

if options is array, list all options separately. Also, rename type from 'block' to 'keyword' (not sure if this is technically correct, but it makes more sense to me):
![screen shot 2014-09-04 at 12 55 21 pm](https://cloud.githubusercontent.com/assets/108094/4153160/80495f1c-3455-11e4-96f1-b24f60561511.png)

if type=function, show examples of all available functions:
![screen shot 2014-09-04 at 12 55 13 pm](https://cloud.githubusercontent.com/assets/108094/4153175/a07d636e-3455-11e4-9763-e021840bd7e9.png)

If type=expression and serialization=content, show examples for both printing arbitrary text and using existing fields:
![screen shot 2014-09-04 at 1 01 21 pm](https://cloud.githubusercontent.com/assets/108094/4153197/d7556594-3455-11e4-95b3-bec54bafc40b.png)

All other type=expression parameters get both an expression and a number example:
![screen shot 2014-09-04 at 12 54 56 pm](https://cloud.githubusercontent.com/assets/108094/4153200/e3ef1a66-3455-11e4-90be-a933ef050543.png)
  
If type=color, show examples: 
![screen shot 2014-09-04 at 12 54 49 pm](https://cloud.githubusercontent.com/assets/108094/4153209/f4debb88-3455-11e4-89e4-d9143cfa3b21.png)

Co-ordinating with updates to Mapnik reference: https://github.com/mapnik/mapnik-reference/issues/67

Todo:
- [x] Add examples for most parameters.
- [ ] Fix tests