feat(uiSrefActive): Also activate for child states.

To limit activation to target state use new `ui-sref-active-eq` directive

Breaking Change: Since ui-sref-active now activates even when child states are active you may need to swap out your ui-sref-active with ui-sref-active-eq, thought typically we think devs want the auto inheritance.

Fixes #818
Merge pull request #927 from angular-ui/issue-818

feat(uiSrefActive): Also activate for child states.