* Resolve specified extras of poetry directory dependencies (#282)

* Copy poetry package extras to directory dependency

* Exclude unspecified extras during version resolution

* Add tests in test_installer

* Pass build on Python 3.5

* Remove Python version in test & fixture

As per
https://warehouse.pypa.io/api-reference/legacy/#get--simple--project--,
the project URL should have a trailing slash.

This was discovered as an issue in an edge case situation,
but should be the standard call regardless.

Edge case explanation:
In a private pypi instance built on artifactory,
set up to serve on https (443) and ignore http (80) requests,
artifactory sends a 301 redirect when called without the slash,
but provides the redirect url as http (due to a bug in artifactory).
This results in an almost-valid call (but missing the slash)
failing by being redirected to a nonsecure call that isn't answered.

For whatever it is worth, there is a Bug filed with Artifactory
for the bad redirect (https://www.jfrog.com/jira/browse/RTFACT-14235),
but it is over a year old with no movement.

* fixed unresolved reference

* fixed code formatting

don't let the user wonder what command to use - just point it to him

* Adding support for HTTP Basic Auth when pulling from legacy repo

* added docs

* Adding unit test for http auth

* Adding mock to dev deps

* moved config reading logic to helper function, adapted test

* corrections

* rework pip_installer

* use source_reference as repo name when writing lock file and installing

Issue #61

* Allow empty license in init's license validation

* Add test for empty license in interactive init