Mscorlib features

[raysplaceinspace]

This pull request just contains additions/fixes to mscorlib:

• .NET AddMonths and JavaScript AddMonths was working differently. Made the JavaScript one work the same as .NET, and wrote tests to cover this. This was quite important for us - we needed to have C# and JavaScript work identically for this project to be useful.
• Addition assignment of TimeSpans to DateTimes wasn't working (it'd do the addition but not the assignment). Fixed that. See the unit test for an example.
• Added some features we were using in C# that weren't already in mscorlib (another overload of ArgumentException, as well as AssemblyInformationalVersionAttribute)