Disallow octal literals in strict mode and ES5

This change alters the way we parse octal literals to match the behavior of the old compiler. We also error now on octal literals in strict mode and -t es5.

There is one breaking change from the old compiler. Namely, in strict mode in ES3 (which doesn't exist in javascript anyway), we will error on octal literals, whereas the old compiler did not.

Next work item is octal escape sequences, which the old compiler did not detect.
V8 allows 009 in strict mode, and treats it as 9.

Perhaps TS should either follow the same behaviour, or generate a warning.