Raise an exception when comparing instance with differing framerates

[nickswalker]

Users should define their own semantics for comparing timecodes that have different framerates, and now they're forced to. Use NotImplemented for undefined comparisons to unknown types to allow other classes the chance to provide an implementation. This allows other class authors to implement comparisons: https://docs.python.org/3/library/numbers.html#implementing-the-arithmetic-operations Closes eoyilmaz/timecode#62