Handle creation of new repositories

This PR adds a new feature that allows for:
 - Perform some checks before creating a new repo;
 - Calling some command when a new repo is created.

# Implementation

When you `gitreceive init`, a new script named `newrepo` is created next to the `receiver` script.

When a new repository is to be created, the `newrepo` script is called instead of simply creating a new repository.

Of course, the default of the `newrepo` script is to add a new repository as always, but it also takes a few other parameters that allow to neat uses of this feature, namely the name of the repository, the username and fingerprint of the user trying to create the repository.

# Use cases

## Performing some checks before creating a new repo.

This allow to control which user/fingerprint can create which repo. Or to inforce some policy on the repo names.

As an example, the following `newrepo` script will make sure that users can only create repo starting with their usernames followed by a dash (note that only the middle part has been edited from the default `newrepo` script):

```bash
#!/bin/bash
# This script gets called to create a new repository at $repo_path.
# Note: /dev/stdout is already redirected to /dev/null.
declare repo_path="$1" repo_name="$2" username="$3" fingerprint="$4"

# Check if $repo_name starts with $username-
if [ "${repo_name##"$username"-*}" ]; then
    echo "Error: your repos should start with '$username-'." > /dev/stderr
    exit 1
fi

# The default is to create a bare repository ($repo_path is absolute)
git init --bare "$repo_path" || exit $?
```

And here is the output when you try to git clone (the same happens if you `git remote add` and `git push` on an existing local repo):
```
$ git clone git@example.com:foobar
Cloning into 'foobar'...
Enter passphrase for key '<snip>': 
Error: your repos should start with 'alice-'.
fatal: Could not read from remote repository.

Please make sure you have the correct access rights and the repository exists.
$ git clone git@example.com:alice-foobar
Cloning into 'alice-foobar'...
Enter passphrase for key '<snip>': 
warning: You appear to have cloned an empty repository.
Checking connectivity... done.
```

## Calling some commands when a new repo is created

Just append those commands at the end of the `newrepo` script.

# Call for reviews

This is quite an important change, but I believe the feature could allow many use-cases, as it is quite flexible. For example, I put the `git init --bare` in the `newrepo` script to allow that to be changed if needed (however, make sure that a `repo_path` is been created).

Please note that if this PR is accepting, current gitreceive users should add a `newrepo` in their existing `$GITHOME`s, since it is only created when you `gitreceive init`. This means that some communication may be needed.

Pleade **do** review this PR. I tried to stick to the coding conventions I saw, but feel free to mention any problem (e.g. if you don't like the name of the `newrepo` script).

Final note: I've put a new link to the wiki on the README, which would have to be populated when/if this PR is merged. The content of this PR can act as a good starting point for that.