libearth 0.2.0

• Session files in .sessions/ directory become to be touched
  only once at a transaction. [#43]
• Added SubscriptionSet.contains() method which provides
  recursively=True option. It's useful for determining that
  a subcategory or subscription is in the whole tree.
• Attribute.default option becomes to accept only callable objects. Below 0.2.0,
  default is not a function but a value which is simply used as it is.
• libearth.parser.heuristic module is gone; and get_format()
  function in the module is moved to libearth.parser.autodiscovery
  module: get_format().
• Added Link.html property.
• Added LinkList.permalink property.
• Fixed a FileSystemRepository bug that conflicts
  reading buffer and emits broken mixed bytes when there are simultaneous
  readings and writings to the same key.
• Fixed broken functions related to repository urls on Windows.
• Fixed libearth.compat.parallel.cpu_count() function not to
  raise NotImplementedError in some cases.
• Fixed Rfc822 to properly work also on non-English locales e.g. ko_KR.