Added the abstract DigestContext base class.
Renamed OmniFoundationErrors.Domain.
Added MD5DigestContext.
Moved the digest classes into Omni.Foundation.Security.
Simplified MD5DigestContext.ProcessBuffer().
Implemented MD5DigestContext.CompleteGeneration().
Fixed the implementation of DigestContext.InitializeForVerification().
Added SHA1DigestContext.

I also globally disabled CS0169. (I tried disabling it only where it
comes up spuriously, but Xamarin Studio kept mangling my #pragmas, so I
had to disable it in the project.)