Added ability to changed prediction time

Use r_oculusrift_prediction cvar to specify time (in ms) to use for HMD
prediction.

Default is 40ms, values less than or equal to zero disable prediction,
and it caps at 75ms.
Allow adjusting HMD prediction time without full reinit of HMD
Enable magnetic drift correction

Uses r_oculusrift_driftcorrect to enable or disable.
Don't enable prediction and drift correction as part of InitOculusSDK()
Merge pull request #2 from dghost/pull

Updates related to OculusSDK