Add update_from_dict
Introduce document views
Introduce views and sources

Views export data from a document whereas sources are data imported
to a document. Both have filters allowing data transformation.
The aliases feature has been removed but it is possible to achieve the
same goal with the rename_field filter builder.
Recursively call the from_<source>

The initial behavior was to call the document constructor whatever the
source. This change makes the import call recursively the from_<source>
function.
Rewrite filter support

A filter can be any callable or a string naming a method of the
document.
Improve documentation and code
Improve views and sources

Create a default view or source on nested documents
Allow properties in sources
Add several tests
Add support for multiple parents and remove slots
Make use of MutableSequence from collections
Add a Mapping field type