Add optional functionality to pre process and extract parameters. 

### Description
The goal is to extract and pre process parameters of the request.

For example Rails, by default, transforms empty array to nil value, for some people it could be better to transform it again to an empty array. 

Or if we want to support a custom parameter type, like enumeration (comma separated values), this could be used to transform automatically the string parameter, representing an enumeration, to an array.

### Usage

To use it, set processing_value configuration variable to true.
```ruby
Apipie.configure do |config|
  config.process_values = true
end
```
In your controller action, use values variable instead of params.

To implement a custom pre processing, the idea is to write a `process_value` function in the validator:
function in your validator:
```ruby
   def process_value(value)
    value ? value.split(',') : []
   end
```

#### as
In the same time, I added an option in parameter description: `as`.
The idea is to add the possibility to have a mapping/separation between names we expose to the external world and the names we use internally.

example:
```ruby
param :client, Hash, as: :user do
  param :default_language, String, as: :locale
  param :name, String
end
```

not processed params:
```ruby
params : { 'client': {'default_language': 'en', 'name': 'toto'} }
```
processed params (available through `api_params`:
``` ruby
api_params: {user:{locale: 'en', name: 'toto'}}
```
What do you think about this functionality in Apipie ?