CDPATH fix, improved CLI help, updated URLs in read-me, cleaned up package.json

Makes sure that CDPATH isn't set, as it causes `cd` to behave unpredictably - notably, it can produce output, which breaks fullpath(). 

Also: Improved CLI help, updated URLs in read-me, cleaned up package.json:

I've tried to clarify the intent of `-x` in the CLI help, but I haven't touched the read-me in that respect.

I don't see any benefit to `-x`:

* Just using `#/bin/sh` as the shebang line in combination with `-s <shell>` gives you the same functionality,
* When it comes to invoking scripts from _within_ test scripts, nothing can do the work for you: you consciously have to mark the invocation with _something_ to indicate that it should be controlled from the outside; it won't get any easier than `$TEST_SHELL ...`  
* Finally, using a shebang line such as `#!/usr/bin/env urchin -x` is problematic for two reasons:
  * Some platforms can handle only *1* argument in a shebang line.
  * In a _package-local_ installation, `#!/usr/bin/env` may not find the Urchin executable.

I'm also not sure how the following (from `readme.md`) fits in the picture:

> It might make sense if you do this.

    export TEST_SHELL=zsh && urchin -x
    export TEST_SHELL=bash && urchin -x

(As an aside: To achieve the same thing, you don't need `export`; `TEST_SHELL=zsh urchin -x`  and `TEST_SHELL=bash urchin -x`  is the better choice.)
How does this relate to use in a  _shebang line_?

`urchin_help()` now uses a here-doc: easier to maintain, and should work in all Bourne-like shells.

`readmeFilename` removed from `package.json`:

> "The readmeFilename does not need to ever be in your actual package.json file" - npm/npm#3573

> I actually hadn't known about CDPATH before. I love strange

That's the problem: it's an obscure feature with - to most - unexpected side effects that introduce subtle bugs.

A way to mitigate the issue is to _not export_ `CDPATH`, so that at least  (non-sourced) shell scripts do not see the variable.

However, _sourced_ scripts may still be affected, so an even more robust approach is to define a function - which you have to source - that _localizes_ setting `CDPATH`, as follows:

```shell
# Safe use of CDPATH localized to a [to-be-sourced] function.
# Note that, in order to protect other sourced code, the function is NOT named *cd*, but *cdp*.
cdp() {
  local CDPATH=~  # sample value
  builtin cd "$@"
}
```