Number and string parsing.

1. Adds number and string parsing in `Parser` class.
2. Adds prefix and postfix expression parsing.
3. Implements `PrefixNode` and `PostfixNode` in `node.cc`.
4. Adds utility class which currently contains number
   parsing routines.
Merge pull request #3 from peelonet/number-and-string-parsing

Number and string parsing.