doc update
Add SignUpdate and SignFinal
Better use toError()
C.free all memory converted woth C.GoBytes
refactor the session stuff for future tests
Add WaitForSlotEvent and make it nonblocking

It returns channel that can be read from the Go side of things.
Sort the functions

Sort all the C function and the Go functions so that they match
the order of pkcs11f.h

This is almost done
Add os
Add GetAttributeValue