Upgrade solution from F# 2.0 + VS2010 to F# 3.1 + VS2013 #103

This is a prerequisite to issue 89 (adding support for .NET 4.5 and SL5).

We'll need to:
1) Upgrade the solution and projects
2) Address any F# 3.0 breaking changes
3) Drop the SL4 projects (which are no longer supported; we will replace with 
SL5 support, see issue 89)
4) Adapt the build scripts (perhaps taking the opportunity to move away from 
.bat scripts, see issue 74)