Openfisca-web-api

* Technical change: return a nice error message to the user instead of a generic "Server Error".
- Impacts `simulate` endpoint.

* Breaking change: Adapt Web-API to OpenFisca-Core 10

* Technical change: return the error message to the user instead of a generic "Server Error".
- Impacts `calculate` and `simulate` endpoints.
- In particular when a wrong period is specified for an input variable in the JSON payload

* Restrict the accepted formats to define a period
- The valid formats are listed in [the doc](http://openfisca.org/doc/periodsinstants.html)

* Improve error messages when the test case is invalid
* Make the web API compatible with `OpenFisca-Core v8`

* Technical and not breaking change
- Move JSON representation of entities to the Core for code factorization

* Adapt the web API to make it compatible with `OpenFisca-Core v7`