Adds server autostart functionality.

[DuFF14]

Note that at this time there is a bug preventing this from working unless an environment variable is changed:

For the environment variable OSVR_SERVER_ROOT, remove the trailing slash in the directory

[DuFF14]

@JeroMiya any issues with merging this before the bug is fixed?

[JeroMiya]

The only issue is that auto-start won't work properly if users run the installer more than once (or I think the latest installer since it adds a leading slash), however once the installer is fixed, the user just needs to run the latest installer to fix existing apps, so it's probably ok to merge this.

[DuFF14]

@JeroMiya I've held off on merging this to make sure that it will work with Android. Keep me updated on any Android progress as it pertains to running Unity apps as I know you've been doing some work there.

[cobbchaseRazer]

@DuFF14 There is currently a larger discussion ongoing about the future of the Android platform. Please hold off on merging more auto-start functionality until we make progress on the discussion. @JeroMiya @leemichaelRazer

OSVR/OSVR-Core#433

[DuFF14]

Restricted this to Standalone Windows platform only. @JeroMiya let me know when the OSVR_SERVER_ROOT fix is in place and then I'll merge. Thanks.