Allow custom config.hdf file and a bit of cleanup

Did a bit of cleanup and refactored so that we can now have a custom
config instead of a default one. The main thing that this solves is allowing custom config files.

- moved functions to `common`
- changed detect to look for `conf/config.hdf`
- `compile` will create the `webroot` folder if necessary
- default config is gone for now, I'll follow up with diffs to fix that
- moved HHVM_VERSION to it's own file. In time we'll probably have a few of these hard-coded values, I don't think they make sense in the main compile script


Tested by creating a simple project:
```
conf/
  - config.hdf 
webroot/
  - index.php
```

Contents for config.hdf:
```
Server {
  DefaultDocument = /app/webroot/index.php
}
```

Contents for index.php
```
<?php

echo "Hello HHVM!"
```

![screen shot 2013-08-27 at 4 20 08 am](https://f.cloud.github.com/assets/821842/1030847/9a626ff6-0ec7-11e3-899b-07725a939ce0.png)