Factor out minimal test to be used as base for custom test

This will allow to start the building of the tests for an application
based on Eve by subclassing the TestMinimal class.
Eve-Mocker by Thomas Sileo added to the extensions page.

Eve-Mocker is a mocking tool for Eve powered REST API, based on the excellent
HTTPretty, aimed to be used in your unit tests, when you rely on an Eve API.
Update to the Eve-Docs extension description
Remove few assert: they are in the TestMinimal class already
Add docstring to TestMinimal
Merge branch 'feature/test-minimal' of https://github.com/ahref/eve into ahref-feature/test-minimal
TestMinimal class added to the test suite
Daniele Pizzolli
Merge branch 'ahref-feature/test-minimal' into develop
Support for MongoDB Write Concern at global and endpoint level.

It is now possible to set the MongoDB write concern level at both
global (MONGO_WRITE_CONCERN) and endpoint (mongo_write_concern) levels.
The value is a dictionary with all valid MongoDB write_concern settings
(w, wtimeout, j and fsync) as keys. {'w': 1} is the default, which is
also MongoDB's default setting.
'_links' now includes a link to the last page (HATEOAS). Closes #62.

Responses to GET requests for resource endpoints now include a 'last'
item in the '_links' dictionary. The value is a link to the last page
available.  The item itself is only provided if pagination is enabled
and the page being requested isn't the last one.
Fix documentation: s/AUTH_USERFIELD_NAME/AUTH_USERNAME_FIELD/
Changelog updated with Julien Barbot documentation fix (PR #64).
Julien Barbot
Merge branch 'klyr-develop' into develop.
Another occurrence of badly worded 'auth_username_field'
Support for manipulation of outbound documents via callback functions.

The ``on_getting``, ``on_getting_<resource>`` and ``on_getting_item`` event
hooks are raised when documents have just been read from the database and are
about to be sent to the client. Registered callback functions can eventually
manipulate the documents as needed.

Please be aware that ``last_modified`` and ``etag`` headers will always be
consistent with the state of the documents on the database (they  won't be
updated to reflect changes eventually applied by the callback functions).

Closes #65.
Cerberus v0.3.0+ is now required. Support for 'float' and 'nullable'.

Cerberus is now a requirement, which enables built-in validation for
float data-type and the new 'nullable' schema setting.
CORS OPTIONS support for resources and items.
CORS Access-Control-Allow-Headers header.

Sent with CORS OPTIONS request. Added X_HEADERS configuration option.