Final release candidate for version 2

Changes compares to beta7:

• the bundle has now good test coverage (tests execute on Travis with eZPublish versions 2014.3, 2014.11 and eZPlatform 1.4)
• compatibility with older (?) Doctrine versions
• documentation fixes and updates
• fix update of contentType identifier (issue #50)
• fix issue with legacy-kernel Slots and db transactions (issue #53)
• improve stability of content deletion by contentType identifier
• allow setting refs on location creation
• make sure root user does not stay logged in if a migration step fails
• fix command to generate role migrations
• add support for references in object-relation and object-relation-list attributes
• fix reference resolver
• add support for ezcountry fields
• remove leftover var_dump calls

The list of changes since version 1.X is at https://github.com/kaliop-uk/ezmigrationbundle/blob/master/WHATSNEW.md