Add a timeout to cmdb pushes / pulls.

Ruby doesn't seem to expose the POSIX alarm(), ergo I did a nasty and dlloaded it from libc.
Other suggestions on how to do this more sensibly welcome, but I can't think of any that don't
involve re-writing significant chunks of the ruby git library, or doing nasty things like forking
a sub process which then sends the SIGALRM if it isn't killed within N seconds...