Improved download button and features grid

Rather than having multiple download buttons and then hiding one of
them, the page now just contains one button, the properties of which are
modified according to the OS being used. If JS is disabled, the download
button will point to the 'Downloads' page.

The features grid used to look a bit odd, it has now been fixed.
Added Downloads page & Decoupled the less file

A 'Downloads' page has been added which contains all available
downloads.The download links are listed in a table, so that in future
we can easily add a new row for a new version etc.

The design of the download page has been taken from:
https://www.apachefriends.org/download.html

The 'styles.less' has been broken into multiple files for easier
maintenance.
Addressing issues raised in the comments

...on https://github.com/musicbrainz/picard-website/pull/2

Changed titles of the pages so that it describes the current page.

Removed some periods and replaced the others by a &middot.

Uncommented the GH Buttons in the footer. I think I should write a
pre-commit hook or something for this ;)
Created a template system using gulp

This is a really basic template system created using the
gulp-file-incude plugin https://www.npmjs.org/package/gulp-file-include

The files that need to be reused are placed in the templates/ folder and
are then included using @@include() in any html file.

The gulp tasks have been largely modified. Now, a temporary build is
generated in the 'tmp/' folder and the webserver now uses 'tmp/' as its
root.

As soon as a file in the 'static/' folder is changed, the change is
reflected in the 'tmp/' folder and the webpage is reloaded.
Merge pull request #3 from musicbrainz/templates

A home made templating system using Gulp and its plugins.
Merge pull request #2 from musicbrainz/downloads_page

Added a Downloads Page