async: checkpoint implementation of async platform
async: xid type is now in OpenFlow_Header

Eventually, remove all aliases to this type.
async: added Message.header_of to 1.0 and 1.3
async: added Async_Platform.Message wrappers

Added wrappers for 1.0, 1.3, and uninterpreted chunks. This allows the platform
to be instantiated with any of these protocols.
.gitignore

vim swap files