Add support and docs for async cachable delegates

[alastairtree]

Add support and docs for async cachable delegates

[jxmiller]

I second this... perhaps?
This compiles:
var result = cache.GetOrAdd("Key", async () => await DbContext.Database.ExecuteStoredProcedureAsync<MyClass>(paramObj, ProcName));

But then it hangs forever.

[alastairtree]

Yeah, ideally we would have GetOrAddAsync methods supporting async/tasks. Will have a look when I get some time.

[alastairtree]

I have made good progress on this and have a working version. See #9

I would appreciate someone to try it out or review the code? Package available for testing from the appveyor build artefacts

[alastairtree]

LazyCache 0.7 now support async/task based caching.