[#382] Remove keys from extras when using convert_from_extras

If fields are moved to the top level, they should be removed from
extras. See #376 for discussion. Added test.

Basically adapt the converter for transforming this:

```
 ('extras', 1, '__extras'): {u'package_id': u'9fbddea7-31bf-4bf2-9595-ae7b5d9c1102',
                             u'revision_id': u'a13e1cf5-6ecb-4ea4-a4c2-f5c0d865e401'},
 ('extras', 1, 'deleted'): <ckan.lib.navl.dictization_functions.Missing object at 0x3c4a050>,
 ('extras', 1, u'id'): u'871cbb3c-8c93-4afa-bf2a-8f2123a7b365',
 ('extras', 1, u'key'): u'frequency',
 ('extras', 1, u'revision_timestamp'): '2013-02-22T17:33:11.577374',
 ('extras', 1, u'state'): u'active',
 ('extras', 1, u'value'): u'DAILY',
```
to this:

```
 ('frequency',): u'DAILY'
```
[#226] Implemented generic text highlighter for JSON, XML and plain text. Deleted jsonpreview.
unified indentation
[#226] Rename text highligher extension to text preview
[#226] Add missing license for highlight.js
[#226] Use github style
[#226] Make jsonp preview more resistant, add missing converter to jsonp
[#226] Rename text preview tests, add test for whether css is present
[#226] Make it easier to add new formats to the json/xml/text preview
[#473] Remove logic.ParameterError because it is not different from logic.ValidationError
[#473] get_or_bust returns an array after the field because to make it consistent with logic.validate
[#473] get_or_bust uses schema validation so that we use common code paths and similar validation procedures
[#473] related_list validates whether we have either an id or a name
[#473] datastore_create should rely on a valid context
Otherwise errors occur when doing a deep copy of the context in validate
[#473] indicate optional parameters in datastore API
[#473] Test that wrong record type raises Integrity Error
[#473] Return IntegrityError as json from API
[#473] related_list should return Query if neither an id nor a name are provided
[#473] Validate parameters for current_package_list_with_resources
[#473] Add offset to current_package_list_with_resources and mark page deprecated