Halfapi

- The definition of "HALFAPI_DOMAIN_MODULE" environment variable allows to
specify the base module for a domain structure. It is formatted as a python
import path.
The "HALFAPI_DOMAIN" specifies the "name" of the module

- Allows to define a "__acl__" variable in the API module's __init__.py, to
specify how to import the acl lib. You can also specify "acl" in the domain's
config

- Add XLSXResponse (with format argument set to "xlsx"), to return .xlsx files

- (rollback from 0.6.12) Remove pytest from dependencies in Docker file and
remove tests
- (dep) Add "packaging" dependency
- Add dependency check capability when instantiating a domain (__deps__
variable, see in dummy_domain)

- Installs pytest with dependencies in Docker image, tests when building image

- Fix "request" has no "path_params" attribute bug