Add support for sub-devices, closes #6

Not yet tested
Add basic support for sub-devices in OCL stub
so that everything compiles. However, clCreateSubDevices()
doesn't do anything yet.
Add complete support for sub-devices in OCL stub
Add sub-device test for PARTITION_EQUALLY partition type
Add sub-device test for PARTITION_BY_COUNTS partition type

Fix bugs in OCL stub related with this type of partitioning.
Update frontpage: remove sub-devices support from to do list