Whitelist only `for` and `class` attributes on BooleanInput label.

I can't find a decent reason to be merging with `input_html_options` and fussing around with it like it was.

Closes #995
Allows to disable model infered class from form
Renames variable, adds tests and documentation
Merge pull request #1016 from justinfrench/whitelist-boolean-label-html-options

Whitelist only `for` and `class` attributes on BooleanInput label.
Configure model class through a proc
Merge pull request #986 from groupbuddies/master

Allows to disable model infered class from form
add failing spec and fix rails version check
use Gem::Version for all version checks
Merge pull request #1023 from justinfrench/fix-rails-version-detection

Fix rails version detection
ensure only one hidden input in rails >= 3.2.13
improve deprecation message and add TODO