Support loading icu libraries on MacOS

[jasonleenaylor]

Using icu.net on a Mac crashes trying to load libdl.so.2.

I'd like to see a NativeMethods solution for Mac which uses libdl.dylib instead of libdl.so.2

[ermshiperete]

Fixed in #142

[ermshiperete]

The nuget package is missing the icu.net.dll.config file, so it's still not fixed in the nuget package.