Merge pull request #39 in G/fastr from...

Merge pull request #39 in G/fastr from ~ADAM.WELC_ORACLE.COM/fastr:feature/r_caller_overhaul to master

* commit 'da57ed19':
  Folded RSyntaxNodeWrapper interface into (now also an interface) RCaller.
  Invalid caller is now a singleton.
  Fixed a copyright check.
  Regenerated tests.
  Traversing parent hierarchy in search for RCaller is no longer necessary at all and RCaller now has a typed representation.
  The mget function can now use functions as ifnotfound argument.
  Traversing parent hierarchy in search for RCaller is no longer necessary for call nodes.
  Fixed a caller of function called via do.call.
  Partial fix to the sys.call problem in the context of S3/S4.