Contribute MemcachedStorageService

[Marvin Addison]

I added the context key-tracking feature Scott suggested along with
some decent code coverage, which I believe makes this a complete
solution.

https://github.com/serac/idp-memcached-storage

I'd like to contribute this to the OpenSAML project today using my
shiny new commit privileges. I believe it should go into the
opensaml-storage-impl module. I'm debating whether to put it under a
memcached package since there are a handful of components.

There is a little wrinkle with the tests; they require an external
memcached server running at localhost:11211. I used a Maven profile to
run the tests that require the external fixture; those tests do not
run by default. I don't expect that's a problem, but I thought it was
worth mentioning.

Please let me know if you have questions or suggestions about this.

Best,
M