- Modify documentation to use the extra dependency in order to install
the recommended markdown package version.

Reverts 2ab91bbaaa565dcb89726b6cac0002c1e0644dc3 and resolves #2626

Competing static site generators integrate the functionality of regenerating
content and serving it into their main executable. In pelican this
functionality used to be in an external script `develop_server.sh` which
resides in the blog base directory. This has the disadvantage that changes in
pelican can break the `develop_server.sh` scripts which will not automatically
be upgraded together with pelican by package managers. Thus, pelican should
integrate this functionality into its main executable.

To this end, this commit removes `develop_server.sh` and adds three command
line options to the pelican executable:

 * `-l/--listen` starts the HTTP server (`-s/--serve` was already taken)
 * `-p/--port` specifies the port to listen at
 * `-b/--bind` specifies the IP to bind to

`--listen` and `--autoreload` can be used together to achieve the same
effect that other static site generators offer: Serve files via HTTP
while at the same time auto-generating the content.

Since the `develop_server.sh` script was removed, pelican-quickstart looses the
`develop` option.

Since the `develop_server.sh` script was removed, the Makefile looses the
`stopserver` target and the `devserver` target is replaced by running `pelican
-l` in the foreground.

Since pelican now offers the `--listen` option, the fabfile uses that instead
of starting the socketserver itself.

Fixes #1971.

The quickstart was worded confusingly - it said "from your project directory", which implied doing a `cd ..` down to the `projects` dir, which would cause `pelican content` to fail. In fact, you need to be in the `yoursite` directory, which is the directory that has the `content` directory in it.