Allow cast booleans

[ixti]

In continuation of #26.
Extends cast option to properly cast boolean values (as they normally received from the web forms)

[puzrin]

@pksunkara , sinse pull requests were frozen for a long time, we maintained our fork and did not provided latest patches. Please, accept this one, because #26 was for integers only, and in real life boolean values happens very often.

After that, we will be able to switch back to your original revalidator branch.

[rlidwka]

What do you think about a flag that would actually modify input to make it valid?

I mean like that:

var input = {test:123};
typeof(input.test) === 'number';

require('revalidator').validate(input, {properties:{test:{type:'string'}}}, {cast:true});
typeof(input.test) === 'string';

So an application will always get a correct type after validation. Does it make sense?