fix requests version in requirements.txt
add a new_response function useful for writing tests

Useful when writing tests for code that is using xivo-lib-rest-client.
factorize tests for command

Like new_response, this is not directly used by xivo-lib-rest-client but
is used by xivo-confd-client and xivo-dird-client, at the time of
writing this.
rename resource_url to base_url
Merge remote-tracking branch 'origin/client-homogenization'