Tests and fix for MetricsFilter

this fixes #711 and extends #713
Refactored tests to reduce code duplication and increase readability.
Changed protection level on session field.
Removed unnecessary extend statements.
Removed unnecessary return statement.
Clarified comment.
Split multi-purposed test and clarified names.
Remove reflection hacks for AsyncAppender.

Now wrapper is not Dropwizard AsyncAppender but logback AsyncAppender. So we can use directly to access the syslog appender.
Upgrading mustache
Merge pull request #755 from reines/upgrade_mustache

Upgrade to Mustache 0.8.17
Merge pull request #730 from jangalinski/711_add_tests

Tests and fix for MetricsFilter on BaseReporterFactory

Fixes #711, closes #713
Merge pull request #747 from arteam/remove_reflection_from_sys_appender_test

Remove reflection hacks in tests for AsyncAppender
Merge pull request #739 from citerus/codecleanup

Assorted code cleanup, no functional changes
Unit tests for HttpConnectorFactory

- Check parsing a custom YAML configuration
- Check the default configuration
- Check building a correct server Jetty connector
Merge pull request #748 from arteam/http_connector_factory_test

Unit tests for HttpConnectorFactory
Upgrading freemarker. Exceptions are now more descriptive so we build the web error message ourselves rather than leaking internal class information. See the final bullet point at http://freemarker.org/docs/versions_2_3_17.html#autoid_145
Merge pull request #756 from reines/upgrade_freemarker

Upgrade to Freemarker 2.3.21