fix(ngShow/ngHide, ngIf): functions with zero args should be truthy

Previously, expressions that were a function with one or more arguments evaluated to
true, but functions with zero arguments evaluated to false.

This behavior seems both unintentional and undesirable. This patch makes a function
truthy regardless of its number of arguments.

Closes #5414
fix($resource): prevent URL template from collapsing into an empty string

if url template would result in an empty string, we should make a request
to '/' instead.

Closes #5455
Closes #5493
docs(\$sniffer): fix minor typo

Closes #5544
Merge branch 'master' of https://github.com/angular/angular.js