Make compile-time config mandatory

[slashdotdash]

The following event store configuration is read from the application environment at complile time:

• serializer
• registry
• subscription_retry_interval

Since these settings are mandatory, preventing the application from compiling if missing, they should be provided as options to use EventStore.

It would still be possible for users to make them environment specific by reading the values using Application.get_env.

[slashdotdash]

Fixed by #217.