i128 support

[TimNN]

This PR implements Rand for i128 and u128. Some notes:

• Should Rng gain a next_u128 method?

  This may be desirable in case a Rng can directly produce 128 bit wide outputs.

• Currently i128 support is enabled using a single feature flag, which may not be the best solution:

  We probably need two different feature flags in the future: one to enable the i128 impls and one for toggling the #![feature(i128_type)] (i128_type will hopefully become stable in the near future, so #![feature(i128_type)] will not be needed but the impls will still need to be disabled for older rust versions without i128 support).

  An alternative would be to automatically enable the features for supporting rust versions using a build script. This would be my preferred solution I think, although I don't know what the policy is for using build scripts like this.

• I'm not completely sure about the travis changes.

Fixes #139.

r? @alexcrichton

(If you want the fixme update in a separate PR, please tell me).

[alexcrichton]

Looks great to me, thanks!