New scheme for managing version

Instead of depending on runtime import of pymssql_version.py,
we store version in pymssql_version.h and compile it into pymssql.pyx
and _mssql.pyx and extract the version for setup.py
Call a configurable `wait_callback` in between dbsqlsend and dbsqlok -
this is a good place to do something like a gevent.sleep() because this
is after we've sent the query to the server and before we've received
the result
Add pymssql_gevent_test.py
Pass socket read file descriptor to wait_callback by using dbiordesc
function from FreeTDS. This allows the wait_callback to do
gevent.socket.wait_read and makes things much greener.
Add test for green query with gevent
Fix failing tests - add gevent dep
tests/test_green.py: Run tests for pymssql.execute and
_mssql.execute_query
Add test for green execution of stored procs with callproc
Add greening to _mssql.MSSQLStoredProcedure.execute()

This makes callproc green.