Add '.editable' support package.

Declares an adapter interface, 'IEditable', for mapping content to /
from a file-like representation.

Provides an implementation for stock 'File' objects.

Adds a configuration directive for registering new adapters.
Add an 'ISource' interface to '.editable'.

Applications can define an adapter to this interface, allowing the
object to be edited as plain text / RST, or else as HTML.

Provide a 'register_source_adapter' config directive for this adapter.
The directive *also* registers an 'IEditable' adapter for the context,
using an implementation which depends on the semantics of 'ISource'.
Note lxml dependency.
moved ISource and TextEditable to a separate package (removes lxml dep)
Merge branch 'feature.editable'