Fix up expressions to work with arbritrary expressions.

Fix up the expression parser to actually work with different expressions.  So now,
it handles both longer expressions (ex 4+5+6) and where order of operations takes
effect (ex 4+5*6 and 4*5+6).  Include tests.

git-svn-id: https://groke.mcmaster.ca/svn/grad/colin/branches/TableTool_javization@9844 57e6efec-57d4-0310-aeb1-a6c144bb1a8b