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 have merged this, and it's now on npm as version 0.0.5. Woo!

On 05 Dec 16:01, Michael Klement wrote:
> > @@ -1,5 +1,9 @@
> >  #!/bin/sh
> >  
> > +# Make sure that CDPATH isn't set, as it causes `cd` to behave unpredictably - notably, it can produce output,
> > +# which breaks fullpath().
> > +unset CDPATH
> 
> > 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 "$@"
> }
> ```
> 
> ---
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> https://github.com/tlevine/urchin/pull/14/files#r21409631