Add constructor for existing Glutin window

[adumbidiot]

Possibly closes #59

Adds and documents a constructor that allows the construction of a game window from an existing glutin WindowBuilder and Eventsloop. I chose to duplicate the code from the default constructor as this new constructor does not try to create a window without multi-sampling implicitly on failure.

In hindsight, this might not be enough to close #59 since this does not construct a game window from a pre-existing glutin window. I can expand/edit this PR if that is needed.

[bvssvni]

I think this is close enough to cover https://github.com/PistonDevelopers/glutin_window/issues/59. We can just reopen the issue if there is a need.

Merging.

[bvssvni]

Thanks!