Add pagination info to get results whatever the HATEOAS status

This is breaking the backward compatibility of collection responses when
HATEOAS is disabled. Collection responses now get always the same response
format. HATEOAS status does only affect the presence of not of the _link
section.

Fixes #355
Fix some failing tests

Sorry about that
Move pagination information fields into a `meta` object
Merge branch 'develop' of https://github.com/nicolaiarocci/eve into pagination-info
Update to Cerberus 0.7.1 for self.document change
flake8 fixes
Docs update for _meta keyword
Changelog update for #364
Merge branch 'rs-pagination-info' into develop
Docstrings update for #364.
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(config): make it possible to init app with empty domain

you can use `app.register_resource` to define your domain but
if you start app with empty you get an exception.
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.