Release version 2.0.0

[2.0.0]

• Changed (breaking) - The way the MigrationDocument's Id is determined. Multiple underscores
  will now combined into one separator to be used between each section of the Id.
  This could be a breaking change if there are migrations named with multiple
  underscores, depending on the version of Raven client and server in use. Some
  versions of Raven ignore multiple separators and others do not. This change
  could cause migrations to be run multiple times in some cases if the ids are not
  changed first.
• Added - Ability to inherit from MigrationAttribute to specify custom migration
  versions.