This class implements a parser for EL expressions.
It takes strings of the form xxx${..}yyy${..}zzz etc, and turn it into
a ELNode.Nodes.
Currently, it only handles text outside ${..} and functions in ${ ..}.
author: Kin-man Chung
Parse an EL expression
Parameters: expression - The input expression string of the formChar* ('${' Char* '}')* Char* Parsed EL expression in ELNode.Nodes