Make last_page? behave consistently with last?

Closes #582.

Also amends next_page and helpers for action_view and sinatra to
preserve the intended behaviour.
Merge pull request #583 from lime/last-page-consistency

Make last_page? behave consistently with last?
Raise ZeroPerPageOperation when total_pages/current_page is called after per(0)

closes #296, closes #585
Add 4.2.0.beta1 to tested Rails versions [ci skip]

Skip tests for now, we can run them on next change.
Run test for AR 4.2 on travis
Do not run tests for AR 4.2 against Ruby1.8
Fix typo [ci skip]
Update CHANGELOG [ci skip]
Say something if the template engine isn't available for theme

fixes #600
Add #out_of_range? method to PageProxy

enhancement for #599.
Refactor action view helper methods

 * if statement + 1 method is easier to read than unless statement + 2
   methods.
 * Put #prenset? for code consistency
Do not display link to previous page if scope is out of range
:flushed:
prev_page method doesn't not have to be too intelligent

We mightn't like this behaiviour as it behaves differently from
Paginator::PageProxy.
Fix wrong code in README

You were setting `a` to a `Fixnum` instead of an ActiveRecord Relation:
```ruby
b = a.page(1).per(20).size
NoMethodError: undefined method `page' for 5:Fixnum
```
Merge pull request #603 from jomo/patch-1

Fix wrong code in README