Implement initial num-traits

[boomshroom]

This implements the first 6 traits from num-traits when the num feature is enabled.

• Bounded
• Zero
• WrappingAdd
• WrappingSub
• FromPrimitive
• ToPrimitive

Aside from the conversion traits and Zero, these all correspond to methods already implemented.

[kjetilkjeka]

Thanks @boomshroom !

As explained in #19 these traits should require some nightly only unstable feature as well, the short explanation why is that num_traits are not considered stable enough to have a public dependency on at this point.

A test (travis job) that verify these unstables features cannot be included on stable would be great as well.

[dzmitry-lahoda]

currently these are stable

[bbaldino]

I've got an updated take on some of these here