Extend supported environment variables set to select default device

With this patch, boost::compute::system::default_device() supports the following environment
variables:

* `BOOST_COMPUTE_DEFAULT_DEVICE` for device name;
* `BOOST_COMPUTE_DEFAULT_PLATFORM` for OpenCL platform name;
* `BOOST_COMPUTE_DEFAULT_VENDOR` for device vendor name.

1. If one or more of these variables is set, then device that satisfies all
conditions gets selected.
2. If such a device is unavailable, then the first available GPU is selected.
3. If there are no GPUs in the system, then the first available CPU is selected.
4. Otherwise, default_device() returns null device.

The hello_world example is modified to use default_device() instead of
default_gpu_device().

(edited from: 
BOOST_COMPUTE_DEFAULT_DEVICE="Intel"
is much more convenient and less error prone than
BOOST_COMPUTE_DEFAULT_DEVICE="Intel(R) Core(TM) i7 CPU 920 @ 2.67GHz"
)