automatically detect number of gpus available

Wraps nvidia-smi to query the number of gpus installed.
copy gpu_detection script when setting up worker's working directory
pass gpu count via stdout instead of exit codes.

Exit codes can be ambiguous.
Fixes for cctools_gpu_autodetect.

Renamed gpu_autodetect to cctools_gpu_autodetect.

Redirected error messages to stderr.

Fixed bashisims.
Removed gpu_info_get debug message.
Merge pull request #323 from btovar/rampantmonkey-gpu_autodetection

Rampantmonkey gpu autodetection
Wrap nvidia-smi (a binary for managing NVIDIA gpus which is shipped with the linux driver) to determine the number of GPUs available.

Incorporated automatic detection in work_queue_worker.