Add getMetersPerUnit instance method to ol.Projection
Add test for ol.Projection getMetersPerUnit method
Use ol.sphere.NORMAL.radius instead of 6378000

to be consistent.
Use correct value to convert from foot to meters

(was using value for converting to inches).
Thanks @twpayne
Merge pull request #290 from bbinet/projection_stuff

Add a getMetersPerUnit instance method to ol.Projection class