Changes main content width (Closes #367)

[DylanGuedes]

• Uses bootstrap container to enclose main content
• Adds dinamic stick footer. (mainly an enhancement for short pages)

[rafamanzo]

Maybe we should take this opportunity to remove this logo (we were kicked out of there).

[rafamanzo]

Are the footer changes related to modifying the main content width? They are indeed nice but keep in mind this comment from a previous PR: #370 (comment)

[rafamanzo]

Does the bootstrap container class provide the width limitation proposed by #367?

[DylanGuedes]

The footer changes are not related, but there's no issue for them and I thought that It was a nice addition - I can separate it in other merge request if you want.

About the width, I preferred to use the default bootstrap container (it isn't 72% I guess), because it works better for multiple resolutions (it's dinamic) than using 72% for every resolution (for a phone, for example, maybe a value higher than 72% looks better).

But I can use the fixed 72% width, if you think that it's better.

[rafamanzo]

If you rebase from master and remove that image from the footer I believe we are good to go.

[DylanGuedes]

Done.

[diegoamc]

Something odd is happening with the cucumber jobs for ruby 2.3.0. It seems poltergeist cannot be found. @DylanGuedes, is this something that happens on your local machine too?

[DylanGuedes]

In my local it looks ok, actually. :/
And I checked that the MRs opened by my friends are also broken for the same reason (something relative to capybara poltergeist missing).