Add parallel requests

This adds ability to perform parallel requests by inheriting from `BaseParallel` instead of `Base`. Set `$this->_feeds` to be an array of feeds and they'll be requested at the same time. If there's a problem with any of the feeds it will be thrown as a general exception in the same way as single feeds, there is no partial completion support yet.

The `Programmes` feed now uses BaseParallel so you can pass it an array of arrays of PIDs:
```php
array(
    array('pdfo42','2342sdf','23dfdf'),
    array('pdfo42','2342sdf','23dfdf'),
    array('pdfo42','2342sdf','23dfdf'),
    array('pdfo42','2342sdf','23dfdf')
)
```
This will result in 4 requests for the programmes feed with these PIDs

If a single request is faked then all requests will return with that fake content, regardless of feed name or parameters. As with fails.


This shouldn't be merged until we've got a PR in RW too responding to the programmes feed changes.