device_id: find camera by serial number

this brings back a feature from the old openni(1): Selecting a camera based
on its serial number.

The procedure is simple: When enumerating all available devices, briefly
open and query each one for its serial number. After that, compare the
given device_id with the available serial numbers and select the matching
camera (if available).
Merge pull request #18 from forouher/feature/indigo/serial

device_id: find camera by serial number