Added support for plain english boolean commands and for boolean_tokens ...

Hello, I made two changes to the project: First, for convenience I replace common English boolean expressions with the proper symbols (e.g. "AND" becomes "&"). Second, in addition to validating the boolean syntax I also wanted to validate each variable against a list of acceptable variables, so I added the concept of the $boolean_tokens array, which provides a clean array of all the variables for further validation, etc.