Only enable solution building and related MSBuild features when compiling JSILc on Windows. This should automatically disable it when people try to build JSILc on Mac or Linux using Mono.
Drop use of Client .NET framework profile in JSIL.csproj
Fix wrong conditional on Compiler references
Change the signature of the JavascriptFormatter.Identifier(string, EscapingMode?) overload so that the second argument is not nullable, and introduce an EscapingMode.None value. The null was causing problems because Mono's C# compiler has a slightly different implementation of overload resolution in this scenario, and there were two valid overloads for (string, null) because I'm a moron.