Allowing arrays to be nested in numeric-key hashes

When params was of the form

    {
      :book => {
        :authors_attributes => {
          :'0' => ['William Shakespeare', '52'],
          :'1' => ['Unattributed Assistant']
        }
      }
    }

And its corresponding filter was

    {
      :book => {
        :authors_attributes => {
          :'0' => [],
          :'1' => []
        }
      }
    }

It resulted in params looking like this

    {
      :book => {
        :authors_attributes => {
          :'0' => nil,
          :'1' => nil
        }
      }
    }

This was due to the way hashes with all numeric keys are treated.
I just added a special case when a corresponding filter exists and is an empty
array.
Merge pull request #121 from simonc/121-arrays-within-a-hash

Permitting arrays within a hash
Add ruby 2.0 to travis

Runs only under rails 3.2
Merge pull request #126 from derekprior/dp-ruby2-travis

Add ruby 2.0 coverage to travis builds
Made the Rails version with whitelisting explicit

A 'later version' of Rails was described, but this confused me a little.
I just made the Rails version explicit. Whitelisting was added in Rails 3.2.
Merge pull request #127 from tilsammans/patch-1

Made the Rails version with whitelisting explicit
Implements require with multiple parameters

Normal use is unchanged.

When used with multiple parameters the error raised in case a
parameter is missing lists all the missing parameters.

Returns the params hash if no required parameters are missing.