Show the builder description in the builder's web status page.
 - add builder's description in the DB
 - update builder.tpl.jade
Fix typo
 - Add migration unittest: test.unit.test_db_migrate_versions_039_add_builder_description
 - pep8/pylint fix
 - make sure the new description col is null
Merge pull request #1312 from delanne/builderDescription

Show the builder description in the builder's web status page.