Stride: Better performance version of fn next

Improve the current Stride, even if it may be replaced by a different
implementation.
Add .travis.yml file
Stride: A simpler, better impl using int offsets

Use int offsets instead of incrementing pointers for the stride iterator
-- it performs better (the .next() method is much simpler).

Also use `int` in from_slice and from_stride.
Merge branch 'master' into gh-pages
Update docs