Argument and Symbol syntax in ReWriteVisitor

The visitArgsNode method adds separators to ArgumentNodes. The
visitSymbolNode method prepends a colon to the symbol name.
Merge pull request #34 from castwide/master

Argument and Symbol syntax in ReWriteVisitor.  The symbol change should be fixed elsewhere (getLexicalName should return :foo or foo: but I will at least add a getLexicalName method to SymbolNode)