Vo

Stable release.

Bugfixes:

* dump nested Value objects
* equal checksum for different objects

* to dict return OrderedDict
* repr method

* Changed the way attributes are assigned to object
* Exception is raised on attempt to delete value by attribute and by key
* Documentation

* Value object is immutable now.
* Access to value by attribute and by key: `value['my_value'] == value.my_value`