Improved Constants

Updated the constant for conversion from radians to degrees to double
precision.
Why would he go to all the trouble of creating a double-precision
vector, and not update the constants to double precision? Also, why did
he even include the d? (Maybe that's not a bad idea)
Added Quaterniond, some methods implemented

I added a double-precision Quaternion struct to the project.
Implementing all of the quaternion methods on my own will be
challenging, but it's made much easier by having an existing Quaternion
class I can directly compare my results to.