Make 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

tests/A nonempty CDPATH should not break urchin.

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+cd ..
+export CDPATH=$PWD
+./urchin -f 'tests/urchin exit code' >/dev/null
+

urchin

@@ -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
+
 fullpath() {
   (
     cd -- "$1"