String Node implementation

- StringInput class is implemented for String node
- The base class String is renamed with AbstractString, because it
creates problem in node instantiation. String class gets registered as
String node, instead of StringInput class because of it's name.
- Implemented buildAST method to compile to correct expression for
string.
- For now migration method is still available on this node, so it will
still be migrated to CBN. Tracked with
http://adsk-oss.myjetbrains.com/youtrack/issue/MAGN-2376
Merge pull request #1402 from sharadkjaiswal/070Countdown

String Node implementation