Added portable support for FSharp.Core

Although I have included Profile88 in the import and PropertyGroup
section it wont build on mono as this profile is not supported.

Only profiles 5,6,14,19,24,37,42,47,136,147,158 for .Net 4.0 and
7,49,78 for .Net 4.5

Conflicts:
	src/FSharpSource.targets