[#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'
```
[#] Only copy the content encoding and type, not all headers to fix issues with chunked responses. Improve overall performance and error reporting.
[#896] Make json preview ajax request async to prevent blocked ui
[#896] Show the original error from the response instead of limited message.
[#896] Require latest httpretty in tests to fix proxy tests
[#896] Do a GET request if HEAD is not supported
Merge branch 'master' into 896-resource-proxy-fixes
[#896] Fix failing test, fix PEP8 issues
[#798] Adds proper background black to IE7 media-overlay
Merge branch 'master' into 382-convert_from_extras-remove-extras
fix for extras not found
[#382] Update failing test
[#972] Do not error if the resource does not exist for datastore dumps
[#972] and [#628] Add docs for datastore dumps again (they have been written for #628 but removed by docs reorganisation).
[#972] After a discussion with @vitorbaptista, we decided to stick with datastore resource instead of datastore table
Merge pull request #972 from okfn/972-datastore-dump-fixes

Datastore dump raises 500 if resource does not exist
Merge branch '382-convert_from_extras-remove-extras'
Merge pull request #798 from okfn/798-homepage-ie7-overlay-bg

Homepage Featured Box Heading Background (IE)
Recompile css
Merge branch '896-resource-proxy-fixes'

Conflicts:
	ckanext/jsonpreview/theme/public/preview_json.js