add writeBlockData method

Hi,
Thanks for making Device::SMBus.

I have used it to make Device::PiGlow and it works great.

I noticed that you implemented the XS code for the block write but didn't expose it in the Perl API,  this implements that.

I've actually implemented it directly in my code, and it works fine as implemented (for what I wanted to do.) So know rush to make a release :)

Because I don't have a device that I can read from I didn't implement the read block data :)