This changes `OscTime` from a type alias into a struct. This allows us
to add implementations to `OscTime`, most notably conversions to and
from `std::time::SystemTime`. This has obvious utility such as
generating an OSC timestamp from the current time or some time relative
to it.

This is unfortunately a breaking change for users of this crate.
Conversions to and from `(u8, u8)` are provided to minimize the impact
of this breakage for users.

Addresses issue #13