Add support for do while loop

[ericdriggs]

@Test
void testDoWhileLoop() {
    eval("a = 0; do { a++ } while(a <= 5)");
    assertEquals(5, get("a"));
}

[ptrthomas]

fixed, thanks for the report @ericdriggs - also this should demonstrate how easy it is to maintain the parser

[ericdriggs]

Thanks.
I agree on maintainability.
I was also surprised how concise the implementation was, and pleased how easy it was to make a failing test case.