Add link to zip download

Adding the complementary part to mkly's changes, making it more friendly for newcomers to know how to get the latest development build without having to know how to navigate GitHub.
Added file description to data returned by the files/get_data tool
set fixed height in slideshow block
determine max height of slideshow images
constant to hide concrete5 version number

as mentioned here http://www.concrete5.org/community/forums/chat/visible-version-number-in-source-code/
don't output unnecessary jquery UI in form block

Take 2 (see https://github.com/concrete5/concrete5/pull/672 for prior discussion).
Also outputs CSS to header (not footer) if needed.
Fix blocktype sort mismatches

Removes gaps in the btDisplayOrder sequence whenever a blocktype is deleted. This prevents future problems when sorting the remaining blocktypes in the dashboard list.
Fix: Login loop with cookies disabled

When trying to login with cookies disabled no error message is shown to
the user. Some 'corporate' networks have this setup by default, I have
experienced it recently with Concrete & a client.

The patch below modifies the login controller to throw an exception on
cookies being disabled.
Close two not closed <div>
Remove extra </div>
Remove extra </div>
Move misplaced start of form
Remove extra </div>
Remove extra </div>
Add missing </div>
Add missing </strong> and removed extra </form>
input check in file download statistic

it's not in issue in the core but if someone uses the method
getDownloadStatistics and allows the user to specify the limit,
we're opening a security hole.
Let's use ValidationIpHelper to detect IP address

The ValidationIpHelper correctly detects visitor's IP address:
let's use it instead of $_SERVER['REMOTE_ADDR']
Improve detection of client IP address

Check for more $_SERVER headers and take just the first IP address if
there are multiple values.
default value added