Report exceptions to Sentry

Summary:
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.

Test Plan:
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.

Reviewers: dereckson

Subscribers: alken-orin

Maniphest Tasks: T764

Differential Revision: http://devcentral.nasqueron.org/D311