Added some more device parameter queries

[MichaelRiss]

I started to implement the clGetDeviceInfo queries.

Two remarks here:

• I changed the return type of the compute_units call from usize to u32 to better match the OpenCL API
• Also, the return type of mem/buffer size calls is u64. This could probably be revised to be usize, but I'm not sure how to handle the situation on an 32bit platform when a 64bit OpenCL device reports memory sizes beyond 4GiB. Therefore I chose u64 for the moment.

Please comment and accept it if you think it's good.