Added description support for migrations

Signed-off-by: Michał Zalewski <michal@mzalewski.net>
Merged with upstream/master
Merge branch 'master' of https://github.com/drucik86/fluentmigrator into drucik86-master
Fix issue with versionloader and fix tests
Merge pull request #401 of drucik86/master

# By Michał Zalewski (1) and Tom Marien (1)
# Via Tom Marien (2) and Michał Zalewski (1)
* drucik86-master:
  Fix issue with versionloader and fix tests
  Added description support for migrations