Added "Daily user registrations" graph.
Daily user registrations graph put in place of cumulative user registrations graph in	index page of statistics.
Merge pull request #11 from nemesisdesign/master

Daily registrations graph
Remove Shellwords.escape for width parameter
added possibility to disable captcha in the account registration form
Hide phone number verification fields if credit card method is pre-selected when receiving a validation error while trying to register a new user.

How to reproduce this issue:
set "credit_card_enabled" key to true in "/configurations"

try to register a new user selecting "credit card" as the identity verification method, leave one of the required fields unfilled so to cause a validation error, then you'll notice the phone number fields are shown but they shouldn't.
Merge branch "master" of @dguerri in "master" of @nemesisdesign
Merge branch 'develop' of git://github.com/openwisp/OpenWISP-User-Management-System into HEAD
Merge pull request #18 from nemesisdesign/master

Merge dguerri and nemesisdesign into a single branch for testing purposes
Merge branch 'develop' of git://github.com/openwisp/OpenWISP-User-Management-System into develop
possibility to disable captcha from configuration key in the database
Merge pull request #19 from nemesisdesign/develop

Enable Captcha DB Config Key
possibility to enable/disable captcha for mobile interface too
Merge pull request #20 from nemesisdesign/master

Finish captcha tweak
Fix issue #23
Merge pull request #24 from nemesisdesign/master

Identity document web interface fix, fixes #23
Merge pull request #31 from openwisp/develop

Merge branch develop into master