Prometheus monitoring for GitLab Pages

Closes #42

See merge request !11

Also fixed the dependencies, moved metrics to its own package and
updated the tests

This starts of the prometheus monitoring for GitLab Pages, and
resolves #42

Point to check:
- Are the metric names good, keeping Prometheus' conventions in mind?
- Golang, general style etc
- Shouldn't I do some voodoo magic to import this in the library?

Allow -listen-http, -listen-https and -listen-proxy to be given more than once

See merge request !6

Don't run lint on go1.5, but run everything on go1.6 and go1.7

Closes #40

See merge request !10

[ci skip]

Per issue #13, sometimes you want to listen on more than one port for each type
of listener. This commit adds support for that.

Add MIT license



See merge request !7

* fix-404-content-type:
  Fix predefined 404 Content-Type
  Add helper for checking if path ends with Slash

Add version flag.

It is handy to have only the version with revision printed out.

Also useful for omnibus-gitlab#1213

See merge request !5

Serve nice 404 page

![Screen_Shot_2016-02-26_at_15.34.10](/uploads/bfedef869fb6f51dec0f3c730069e42e/Screen_Shot_2016-02-26_at_15.34.10.png)

See merge request !4

Present user-provided 404 pages

If the 404.html is present in root of project it will be presented instead of generic 404.

**Example:**
1. If you use the project page (served under /project/) and try to access: `/project/non/exsiting/file` we will try to serve: `/project/404.html`, then `/404.html`.

1. If you use the group page (served under /) and try to access `/non/existing/file` we will try to serve: `/404.html`.

1. If you use custom domain and try to access: `/non/existing/file` we will try to server `/404.html` only.

cc @axil 

Fixes https://gitlab.com/gitlab-org/gitlab-ee/issues/318


See merge request !2