HTML5 Form Validation Attributes

Added
* minlength
* autofocus
* pattern
* autocomplete
readme update

Explanations added for min/max length HTML5 form validation along with
autofocus, autocomplete, and pattern
Fixed lexical error in translation
No grouping in backends contentype listing

I think, grouping should be switched off in backends contenttype listing when another sort order is seleceted by clicking in the header line. What do you think?
More flexible Setcontent parameters in twig
urlencode() href'd attribute strings...  Nothing to see here ;-)
One too many
Custom listing should use slug instead name as filename
Change a silly Dutch label to english.
Merge pull request #1113 from cdowdy/master

HTML5 form validation additions to SimpleForm
Merge pull request #1116 from SilverClash/master

Fixed lexical error in translation
Merge pull request #1122 from rarila/patch-1

No grouping in backends contentype listing
Merge pull request #1123 from rixbeck/feature-1120

[Feature] Allow Setcontent tag accept expression for some attributes
Merge pull request #1124 from GawainLynch/hotfix/Socialite

urlencode() href'd attribute strings... Nothing to see here ;-)
Merge pull request #1125 from rarila/customlistings

Custom listing should use slug instead name as filename.
Bumping version to 1.6.0 beta
Tweaking Travis.yml, see if it works better with submodules
Switching from "ssh" to "http" for the base-2104 submodule. See if Travis likes that better.