Fix Unreliable Hound.Helpers.Session API

[afjackman]

Changes are summarized in https://github.com/HashNuke/hound/issues/113

• Remove ability to name sessions (they have an ID already)
• Remove tie between sessions and processes
• Change in_browser_session to perform_in_session
• Expose active_sessions with function in Hound.Helpers.Session

Changelog will be needed

[Kukunin]

Sessions API seems pretty broken, it seems this PR might be a good starting point for rework:

• #145 can't transfer session between processes (to use browsers pool, for example)
• active_sessions returns different info from what can be used
• hound often leave sessions unclosed (after active work, there were 26 orphan sessions left in chromedriver), haven't looked too close for this.

[danhper]

Hi @Kukunin, thanks for the comment. Unfortunately I don't have time to work on it but I would be happy to review PRs. Thanks.

[Kukunin]

@danhper cool. thanks for the answer. It gives hope that PRs won't be abandoned =) I'll try to send a couple of PRs in the nearest future

[phaer]

I've tested this PR after rebasing it on the current master branch and it continued to work fine while providing an easier API, in my opinion.

Is there anything blocking this from getting merged?