Add Prawn::View mixin for custom documents

Squashed commit of the following:

commit 1a17128177d8978d3ab587fffab9efa33b219c03
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Aug 11 10:59:27 2014 -0400

    Add a manual entry for Prawn::View

commit a95fc344e93d2027f61c79516f810140baeba695
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Aug 11 10:41:00 2014 -0400

    Add test coverage for Prawn::View

commit 443567cc7785da0a8985f6c3b2240fb49766c4de
Merge: ce5eedd 3818c4f
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Aug 11 10:08:49 2014 -0400

    Merge branch 'master' of github.com:prawnpdf/prawn into view

commit ce5eedd734a6bbecf9d71fb51b4395020812ee01
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Aug 11 10:06:48 2014 -0400

    Add API documentation for Prawn::View

commit 0e90868b6a351060d83cd7295cfc6642878eec98
Merge: f7bdf09 12ccc2c
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Mon Aug 11 09:32:25 2014 -0400

    Merge branch 'master' of github.com:prawnpdf/prawn into view

commit f7bdf090b56f17a1d9ebf2dad543a368982e4ed9
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Thu Aug 7 12:02:12 2014 -0400

    Bump to latest Ruby 2.1 build

commit db67e2748eb85c858c6fb632ce119d13736d0ff7
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Thu Jul 31 07:01:22 2014 -0400

    Somewhat softer method_missing hook for Prawn::View

commit 06dec835b9fe4204267f234e0011f29805eb5ebf
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Wed Jul 30 19:37:44 2014 -0400

    Allow NoMethodError to bubble up.

commit afa0ef67f0ba8cabeb435fe757c820b59d9c51b4
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Wed Jul 30 08:41:48 2014 -0400

    Add missing encoding comment

commit 8ce3eaaaa1b81d28acee8dd267b0404d16f0c527
Author: Gregory Brown <gregory.t.brown@gmail.com>
Date:   Wed Jul 30 08:33:52 2014 -0400

    Add Prawn::View