Add support for views

Use parent in views (thanks to @PuckCh).
This could be used for example to set something like:

    "script": "ctx._parent = \"my-parent\""

Add scripts for views (thanks to @PuckCh).

Using script_view, you can make some scripting base on the view result.
For example, you can do something like:

    "script_view": "view._parent = view.myparent"

to fill the _parent field using the variable myparent from the view.

Relative to #2
Closes #24
Add script support for view_keys to update multiple/different view results from one couchdb change