Add support for namespaced input classes

Hi there!
First things first - thank you for this awesome project.

I found that using global namespace for custom inputs does not fit our applications, so I made this patch which allows you to namespace custom inputs into custom namespace (form builder class by default).

for example:
```ruby
class CustomFormBuilder < Formtastic::FormBuilder
  class StringInput < Formtastic::Inputs::StringInput
  end
end
```
Will use `CustomFormBuilder::StringInput` instead of formtastic one.

To not break backwards compatibility top level ones are still used first.

Also in the process when writing tests I found out that input helper was not tested for cases when input_classes are on/off so I refactored tests to shared example and let it run for both cases.

Also when doing it I set up some basic mock objects so input helper test can be ran without whole form builder which makes the tests much simpler.


* namespaced input classes can be in form builder namespace
  * the namespace can be overridden in the form builder
* upgrade RSpec to get constant stubbing and other goodness
* refactor part of input helper spec
  * to test behaviour when cache_classes is on/off
  * to not rely on form generation

_edit:_
I added second commit which allows inheriting input classes from parent form builders.
Unfortunately this feature fails on ruby 1.8. Do you have any ideas how to solve it? Mark it as 1.9  & up only?

_edit **2**:_
I refactored loading code to own class called `Formtastic::ClassFinder` which encapsulates all the logic shared between ActionHelper and InputHelper. Also moved tests to own spec file.

_edit **3**:
Added support for custom actions. See test of 287a0e9. Allows to create own actions like: 'DestroyAction' instead of trying to find class nil.