Fix definitions of operators on js types.

This basically solves soundness issues by removing a lot
of definitions from js.Any, and move them down to specific
subtypes. For example, && and || are restricted to Booleans,
and unary + - ~ are restricted to Numbers.

Although JavaScript guarantees that +x will return a number
for any type of value x, Scala.js should not consider this
to be type-safe. Note that +x is still allowed on js.Dynamic,
and returns a js.Number. The same reasoning applies to other
operators.

This also adds the comparison operators to js.Number and
js.Dynamic.
Merge pull request #210 from sjrd/fix-js-operators

Fix definitions of operators on js types.