Add UMD support (AMD, CommonJS, browser globals), add retina support - fixes #215, fixes #257

Note that the tests are broken, and were already broken before I made any modifications:

```
Running "jasmine:src" (jasmine) task
Testing jasmine specs via phantom
...................................................>> InvalidValueError: unknown property error at
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:12 
>> :8 nO
>> :5 mO
>> :11 
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:25 Q
>> :11 
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:25 
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:25 
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:10 K
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:25 Qf
>> http:/maps.gstatic.com/intl/en_us/mapfiles/api-3/14/11/main.js:25 Uf
>> :55 
xx.....
Drawing a route:: should add a line in the current map: failed
  timeout: timed out after 500 msec waiting for The drawn route should create a line in the current map (1)
Getting routes:: should return an array of routes: failed
  timeout: timed out after 500 msec waiting for #getRoutes should return the found routes as an argument (1)
58 specs in 1.971s.
>> 2 failures
Warning: Task "jasmine:src" failed. Use --force to continue.
```