WIP mailer function
Added rake task to send out mailing to users

This rake task is used to send a mail to all users who have indicated that
they like to receive updates regarding the ETM. The text of the mail has to
be stored as a template in `app/views/user_mailer`, either in html or text
format or both. Apart from the name of the template to be used (w/o extension),
one needs to provide te subject of the mail. No e-mails are sent unless force
is set to TRUE.

Example:

`rake send_mail subject="Test mail" template="test_mail" force=TRUE`
Only send to users that have scenario(s)
using Raise in stead of puts
making English Version less prominent
Fixed checking for existing templates. Closes #1467
Eliminated chdir and used string interpolation
Merge pull request #1464 from quintel/mailer

Added rake task to send out mailing to users
Extended README on how to create a new D3 chart type
Added legend for tracker line

Added legend for tracker line, included associated translations of
legend text. Closes #1460.
Changed has_energy_storage into has_electricity_storage

In order to be consistent, this dependable attribute has been renamed to
has_electricity_storage. This change is associated with similar changes
(and commits) to respectively etengine and etsource.

CAUTION: Following the update of these three repositories, the dependent on
attribute of the associated sidebar and chart needs to be updated as well!!
Updated /javascripts/d3/README.md
Merge branch 'labels'

Conflicts:
	public/javascripts/i18n/translations.js

Closes #1471
Move target line out of viewport if no target set

If no target is set (present_value() or future_value() equals null), no target
line is visible as it is moved out of the viewport. If the target is set to
0, the target line is displayed as expected. Closes #1452.
Move target line out of viewport if no target set

If no target is set (present_value() or future_value() equals null), no target
line is visible as it is moved out of the viewport. If the target is set to
0, the target line is displayed as expected. Closes #1452.
Added redcarpet to Gemfile
Render scenario description with RedCarpet to preserve line breaks

In order to preserve any line breaks that may be part of the description
provided by a user, it is rendered as markdown with RedCarpet.

Closes #1473.
Added translations for new slide 'demand_households_population'
Closes #1489
Closes #1492