Change LatestChanges to support a start parameter

This changes the LatestChanges model so that the 'find' method can be 
called with a 'start' argument. 

When the 'start' argument is provided, we pass this into the query made 
to Rummager. When it isn't provided, we default to zero. (this maintains
the existing behaviour)
Change SectorPresenter to support a 'start' arg

This changes the SectorPresenter to support a new argument providing
the offset/'start' value for the list of recently changed documents.

The 'start' value is then passed as an argument to `LatestChanges.find`
where, as part of the behaviour added in the previous commit, it is 
included in the search query to Rummager.

This includes a new test helper for the integration tests, which creates
a stub for Rummager only invoked when the correct 'start' parameter is 
included.
Include pagination metadata in API response

This changes the SectorPresenter so that the values of the 'start' 
and 'total' keys in the latest changes response from Rummager are 
exposed in the API response.

We will need to access these values in order to correctly link to the
next and previous result pages, and to determine whether are additional 
pages or not.
Parse the start value more robustly
Support the 'count' parameter

Allow a 'count' to be specified when requesting the list of latest
changes for a sub-topic. 

Like 'start', the value of 'count' is used to build the request to 
Rummager. 

We're restricting the value to be greater than zero and less
than or equal to 100, which matches the restrictions in Rummager. We 
continue to default to a count of 50 if the parameter is missing, blank, 
negative or zero.
Merge pull request #12 from alphagov/paginate-latest-feed

Support the 'start' and 'count' parameters to paginate the latest feed