Adds ReadContext/WriteContext methods to endpoints and to streams. Update rawread example tool to use ReadContext for implementation of "timeout" flag.
* Store a reference to libusb implementation in the context, transfers
and some more places. Remove the global libusb variable.
* Parallelize tests.
* Fix the link in README.
* add alloc/free_transfer_and_buffer. Manages the buffer memory on the C
side.
* switch libusb.go to use the new alloc/free_transfer_and_buffer. Add
a new buffer() call to get access to the allocated buffer as a Go slice.
* Fake USB transfer uses the new alloc/free/buffer interface.
* Switch to the new libusb.alloc signature, where libusb owns the buffer.
* newUSBTransfer now allocates a separate buffer, do a copy on
endpoint.transfer.
* newUSBTransfer will now allocate it's own buffer.
* Enable autodetach in rawread.
