Misc functions

Hey!

This pull request includes a number of commits. Each commit contains one feature, so you can choose what you want to include.

There are a few things that I was not 100% sure how I wanted to solve, so I figured I would discuss it with you.

* `ht-select` - Currently returns a list. Should it return a list or a HT?
* `ht-reject` - Currently returns a list. Should it return a list or a HT?
* `ht-delete-if` - Currently returns `nil`. I did this because you have done so in this library, but I would prefer to return the table.
* This is more an FYI. I tried to use `ht-select` in `ht-reject` to avoid duplication, but I got endless recursion. Not sure why.
* It's really easy to set up Travis. See [f.el](https://github.com/rejeep/f.el/blob/master/.travis.yml) for a working configuration
* I just added the functions, one after another. Not sure if you want some specific order.