[MODCLUSTER-409] added VERSION method to mod_manager

when mcmp clients are coded, it needs to know what version of mod_cluster is used, and there is no apparent way to get that right now, so VERSION method would expose that data:

```bash
$ curl -XVERSION http://127.0.0.1:6666
```
```
mod_cluster/1.3.1.dev
```

```bash
$ curl -XVERSION http://127.0.0.1:6666 -H "Accept: text/xml"
```
```xml
<?xml version="1.0" standalone="yes" ?>
<version>mod_cluster/1.3.1.dev</version>
```