Initial commit

- Update ReadMe, Contributing and Licence files and some minor changes
in order to use existing code in Espersa project
- Add shell script in order to update local Espersa repository from
upstream Spree repository
Update gems specifications and Gemfiles

- Rename all spree_* gems to espersa_*
- Add Espersa author, email and homepage to gems specifications
- Some minor changes, including update description and summary in some
gems specifications