ADDED: missing OpenAL method 'alSourceRewind'

This pull request adds a missing method to the openAL library, namely 'alSourceRewind'.

> alSourceRewind applied to an AL_INITIAL source is a legal NOP. alSourceRewind applied to a AL_PLAYING source will change its state to AL_STOPPED then AL_INITIAL. The source is exempt from processing: its current state is preserved, with the exception of the sampling offset, which is reset to the beginning. alSourceRewind applied to a AL_PAUSED source will change its state to AL_INITIAL, with the same consequences as on a AL_PLAYING source. alSourceRewind applied to an AL_STOPPED source promotes the source to AL_INITIAL, resetting the sampling offset to the beginning.

Check out the spec for more info: http://www.openal.org/documentation/openal-1.1-specification.pdf

As this is the extension of library code, no tests have been run. However, we have built and tested two games with this patch.