Implement Support for no_std

[CryZe]

num-traits just added support for no_std, so vek can now support no_std as well.

Resolves #20

[yoanlcq]

Looks great! I think it's a good opportunity to add a no_std CI target, as suggested by @cbiffle in #20 ; quoting :

Also, consider adding a no-std target to your continuous build to catch regressions here. thumbv7em-none-eabihf is a supported target on stable and nightly and would be a reasonable choice, but in general, any target with -none- in the triple instead of e.g. -linux- should do.

There are a lot of commented targets in the current .travis.yml; it should be enough to enable at least one of these that's no_std, whichever is most convenient/common I guess :)

Thanks a lot for this work.

[yoanlcq]

Sorry for the delay, I kind of waited for changes in .travis.yml as part of the PR, but then time passed and I just forgot about it.
So at long last, let's merge, and I guess I'll add a no_std CI target myself! Thanks again.