We provide a new service provider, SentryServiceProvider,
bound to the 'raven' application container key, to provide
a Raven client.

The clien will connect to Sentry through a DSN configured in
SENTRY_DSN environment variable.

The exception handler sends exceptions to Sentry,
but not for unit tests.

Fixes T764.

Tested with the sentry.nasqueron.org installation (see T684).

The test procedure involved to check Sentry received something
in the following scenarii:

• fire exception through a new HTTP route
• explicitely call Raven facade through php artisan tinker
• allow unit tests to report exceptions

Unit tests added for the service provider and the facade.