Exception logging

See merge request !54

Add messages to exceptions and slightly refactor exception handling.

Exceptions are now always passed to the logger. The logger decides based
on configuration whether and how they are logged. Logging messages were
adjusted for consistency.

Improve exception handling

See merge request !52

Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

Error responses now expose exception details.

Exeption handling was broken by
56fe085e (GH-27).

Fix session import

See merge request !49

Readd an empty constructor for deserialization with Jackson. It was
accidentally removed a part of a code clean up in commit
685695a6.

Code cleanup

See merge request !48

Additionally, removed unused empty constructor

Additionally, made first use of it in CouchDBDao.

Additionally, remove unused loggers.

Bump Java source and target versions to 1.8

See merge request !47

SessionServiceTest.testShouldDeleteAllSessionData fails when run on
JDK 8.

This will allow us to use type annotations, lambdas, etc. Documentation
already states Java 8+ as a requirement.