v0.2.5.1

• Update: Version.__repr__ results with a more readable string.
• New: Added a generalized generator called
  stalker.models.walk_hierarchy() that walks and yields the entities over
  the given attribute in DFS or BFS fashion.
• New: Added Task.walk_hierarchy() which iterates over the hierarchy of
  the task. It walks in a breadth first fashion. Use method=0 to walk in
  depth first.
• New: Added Task.walk_dependencies() which iterates over the
  dependencies of the task. It walks in a breadth first fashion. Use
  method=0 to walk in depth first.
• New: Added Version.walk_hierarchy() which iterates over the hierarchy
  of the version. It walks in a depth first fashion. Use method=1 to walk
  in breadth first.
• New: Added Version.walk_inputs() which iterates over the inputs of
  the version. It walks in a depth first fashion. Use method=1 to walk in
  breath first.
• Update: stalker.models.check_circular_dependency() function is now
  using stalker.models.walk_hierarchy() instead of recursion over itself,
  which makes it more robust in deep hierarchies.
• Fix: db.init() now updates the statuses of already created status
  lists for Task, Asset, Shot and Sequence classes.