Created .travis.yml.
Removed OTP release R16B01 from list.

Not available yet, so much for future-proofing.
Added Travis CI build status image to README.md.
Updated README.md; made badge a link to Travis CI.

[skip ci]
Integrated Travis CI into v0.2 branch.
Created extension to the list module.

The `foldli/3` function is inspired from the way `array:foldl/3`
operates, as well as the existence of `List.mapi` in the F# language.
Refactored implementation of sending publisher pids.

Now uses `ydb_lists_ext:foldli/3` to keep track of the index for a
particular adjacency pair within the entire topology.

Also moved handler logic for whether the ready pid is on the sending or
receiving end to private functions.