Attribute inspection

When using the plugin with more than 10 locales, I found myself writing duplicated code for each locale. I changed the code to expose the locales per attribute and the translated attributes, so that I can now write this in my code:
```
serialize_attributes :json, *translated_attributes_for(:value)
```
instead of:
```
serialize_attributes :json, :value_en, :value_fr, ... , :value_es
```
which is brittle because it hardcodes the locales in the attribute names.

This change also improved the readability of the implementation.

I elaborated the read me and I fixed a mistake in the read me.
I also added the license file, which was missing but mentioned in the read me.