Issue #352: additional installation checks

* Check that PHP is installed
* Use curl to download data snapshot if mac, use wget otherwise
* minor style cleanups
* exit 1 codes + use echored if errors
We could also just check if wget is available, if it's not fallback to curl?

```
if ! hash wget 2>/dev/null
then
    wget http://transvision.mozfr.org/data.tar.gz -O - | tar -xz
else
    curl http://transvision.mozfr.org/data.tar.gz | tar -xz
```
Comment should go 2 lines up. Rest looks good to me.