Support postfix expression.

Also, fix a bug in variable declarations.  If there was no
keyword (i.e., there was a type but no `var`, `final`, or `const`)
then the keyword and type were both omitted.  This is fixed to emit
`var` as a workaround, though the type should be emitted instead.
Merge pull request #34 from kmillikin/cascades

Support postfix expression.