Added property-based bootstrap for users of xml-based MyBatis bootstrap and added stub for an integration test.

Integration tests should be moved to a submodules, so they don't pollute
the main code base... having them in isolation should also help to notice
when we're doing something stupid that breaks the users.

Property-based bootstrap is to be used when no dependency injection is
available and when user doesn't construct MyBatis' configuration by hand,
from Java code... so it will cover A LOT of users.

This module reads system property 'metrics.registry.provider' that should
contain fully qualified class name of an implementation of
javax.inject.Provider<MetricRegistry>. This implementation is instanitated
using no-arg constructor and is used to, suprise suprise, get
MetricRegistry instance and use that to initialize InstrumentingInterceptor.
Reworked unit tests, finished one integration test, reworked deriving metric name. Still needs cleanups through.

- Reworked deriving metric name because the Invocation#target and
  Invocation#method pointed at executor, not at the actual mapper (it
  makes sense from framework internals perspective in the hindsight...)
- Added `Plugin.wrap(target, this)` calls to the interceptor, based on
  some random comment on the internet - who needs documentation anyway? :P
- Finished integration tests for XML file-based bootstrap
- Updated logging settings for tests