[System.Core] ReaderWriterLockSlim.cs now builds with mono.

* Builds without [MethodImpl(MethodImplOptions.AggressiveInlining)]
  when target framework version is lower than 4.5.

* Use Thread.Volatile instead of Volatile.Read when target framework
  version is lower than 4.5.
Merge pull request #2 from esdrubal/readerwriterlockslim

[System.Core] ReaderWriterLockSlim.cs now builds with mono.
Removed unnecessary call to Thread.WriteVolatile.
System.Threading.Volatile is internal on framework versions before 4.5.
As corlib internals are visible from System.Core we can use Volatile.
Remove unused private using
[System.Core] Disable use of CAS.
The added ifdef is required to circumvent a compile error on older versions of mcs.
componentmodel tweaked to work with mono
Disable UseCompatibleTypeConverterBehavior on mono
Exclude security code from the mobile targets.

Mobile doesn't support any of the security features of .NET, no point in
running the code and/or defining fake types.
[TPL] Add mono internal API support
[TPL] Remove dependency on AsyncMethodBuilderCore
[TPL] Fix object null reference exception
[TPL] Fix not implemented exception