Update to Cerberus 0.7.1 for self.document change
Standardize insert (bulk or not) to comply with RESTful status behavior

For bulk and non-bulk inserts, response status now always either 201
when everything was ok or 400 when something went wrong. For bulk
inserts, if at least one document doesn't validate, the whole request
is rejected, and none of the documents are inserted into the database.

Additionnaly, this commit adopts the same response format as collections:
responses are always a dict with a `_status` field at its root and an
eventual `_error` object if `_status` is `ERR` to comply with #366.
Documents status are stored in the `_items` field.

Example:

```json
{
    "_status": "ERR",
    "_error": {
        "code": 400,
        "message": "Insertion failure: 1 document(s) contain(s) error(s)"
    },
    "_items": [
        {
            "_status": "OK"
        },
        {
            "_issues": {
                "email": "required field"
            },
            "_status": "ERR"
        }
    ]
}
```

This unified response format will ease client implementation.
fix(datalayer): make datalayer usable outside request context

fix when using datalayer outside request context but within app one
Merge branch 'standardized-bulk-inserts' of github.com:rs/eve into rs-standardized-bulk-inserts
Python 2.6 compatibility
flake8
Standardize inserts (bulk or not)
Merge branch 'rs-standardized-bulk-inserts' into develop
Merge branch 'fix-datalayer-app-context' of github.com:superdesk/eve into superdesk-fix-datalayer-app-context
Data layer usable outside of request context.
Merge branch 'superdesk-fix-datalayer-app-context' into develop
Fix ?version=all/diffs bug preventing full response
Fix ?version=all/diffs bug preventing full response
Merge pull request #377 from joshvillbrandt/version_all_diff_bug

Fix ?version=all/diffs bug preventing full response
Merge branch 'develop' of github.com:superdesk/eve into superdesk-develop
Start app without any defined resource.
Merge branch 'superdesk-develop' into develop
itsdangerous 0.24 Closes #378.
Merge branch 'develop' of github.com:joshvillbrandt/eve into joshvillbrandt-develop
Cerberus 0.7.1 is now required