Watch node implementation for DS Nodes

1. BuildAst methods just returns a simple assignment for the input node.
2. Registers PropertyChanged notification for this node.
3. When "IsUpdated" property of this node is changed, it dispatches the
AstNode processing for it's input node.
4. Using MirrorData it populates a WatchNode instance for the given
node.
Merge branch 'ChocoButter' of https://github.com/ikeough/Dynamo into ChocoButter
Merge branch 'ChocoButter' of https://github.com/ikeough/Dynamo into ChocoButter
Merge branch 'ChocoButter' of https://github.com/ikeough/Dynamo into ChocoButter
Ensure that watch and watch3Dd nodes are available

Added [IsDesignScriptCompatible] attribute on Watch and Watch3D nodes,
so that they are available in library view and search.
Updated Math.dll with static class Math

So that no constructor is shown in Dynamo.
Merge pull request #861 from sharadkjaiswal/ChocoButter

Watch node implementation for DS Nodes