Add POST notification to gitaly on post-receive

added 3 commits

77b0d85b...f61bdfe3 - 2 commits from branch master
a759f47a - Add POST notification to gitaly on post-receive

added 2 commits

68ee332b - Separate HTTP logic from GitlabNet
8cf6031e - Add POST notification to gitaly on post-receive

@jacobvosmaer-gitlab this looks good but I don't have a way to test it with gitaly. @ahmadsherif was telling me we don't have real handling of HTTP requests in Gitaly's server yet; I don't know if that's something that I have to do as a part of gitaly#37 (closed) or if that's a separate issue.

@eReGeBe to complete gitaly#37 (closed) we need to also have the endpoint. But we can merge the two changes independently.

Next steps for this discussed in meeting:

Add routing in gitaly (see https://gitlab.com/gitlab-org/gitaly/issues/41).
Ensure that shell works properly if gitaly is not available/fails (log error if it occurs but don't tell the user).

Sounds good. Also: make sure there is a short-ish timeout on this request, maybe 5 seconds.

added 4 commits

2e1299cc - Separate HTTP logic from GitlabNet
c7a788cc - Add POST notification to gitaly on post-receive
097aeb2c - Correctly encode post-receive notification parameters
945b7245 - Be more permissive with Gitaly's connections (for the moment)

Compare with previous version

mentioned in issue gitaly#37 (closed)

mentioned in merge request gitaly!36 (closed)

@jacobvosmaer-gitlab @eReGeBe Should we consider moving GitalyClient to a separate gem in order to be reusable between gitlab-ce (for stuff like https://gitlab.com/gitlab-org/gitaly/issues/48) and gitlab-shell?