Added device extensions to info.

This change adds a new string to the ocl_device_info_t data structure:
device_extensions.  This will be filled with a space-delimited string of
the extensions supported by a device.  Will add a convenience function
for checking specific features in a future commit.
Added convenience function for checking supported extensions.

Example usage:

if(device_has_extension(OCL_DEFAULT_DEVICE,
	"cl_khr_local_int32_base_atomics")) {
	fprintf(stderr, "Device supports base int32 atomics\n");
} // if
Removed test output from example.