added nuget build support for Akka.Cluster

Modified the `build.fsx` file to use a function for selecting the NuGet version number - all it really does is make sure that a -pre suffix is used on specifically named pre-release packages.

At present, here is what the output from `build NuGet` looks like:

![image](https://cloud.githubusercontent.com/assets/326939/4656153/ecd1f9aa-54d0-11e4-8a75-12f1209456f3.png)

@HCanber @rogeralsing do the names of the logging packages look correct? These changes were applied after PR #465 was merged in.

CC @smalldave removed the TODO regarding MD5 for the VectorClock - your original code is inline with the original Akka source and is fine as-is. No need for Murmur3.