New markup for download tab

Add
- episode image
- episode title
- published date
- duration of media file
- download button
- selectbox for file formats
Apply new classname 'button-submit' to button
Rename 'button-subscribe' to 'button-submit',
to generalize the classname for a better modular usage
Add overlay images for download-tab's episode image
Delete commented-out codeblock in _mixins.scss
Apply correct font-family for <h1> tag in 'tabs'

Add <span> to headline text, otherwise the icon-font will be applied on
text, instead of Roboto font.
Specify flexbox-hack to avoid unintended behavior

Delete universal selector * for flexbox hack (padding:1px).
Apply styling only on flexed elements.
Rename class .image-container to .poster-image
Add general styles for <select> elements in_base.scss
Changes in typography

- Change gray color variables (names and values)
- Import and apply new font-families ‘Roboto’ and ‘Droid Sans Mono’
- Delete unused font-weights
New markup and styles for download-tab

- Add poster image with overlay-image
- Display media data like duration and published date
- Add footer with download button and file-format select bar
Set correct return types in comment blocks in download.js
Delete forgotten debug comment
Set fixed width for episode image in download tab
Rename var $listElement into $tabContent in download.js
Merge pull request #187 from podlove/new-download-tab

New download tab