Added solution and project configs for additional portable profiles supported by F# 3.1.2.
ExtCore now requires VS2013 or newer to build; this simplifies the project files quite a bit and also allows to start using F# 3.1-specific features. However, ExtCore still targets the F# 3.0 runtime (FSharp.Core 4.3.0.0) so it can be consumed from VS2012 projects.