feat($templateRequest): introduce the $templateRequest service

This handy service is designed to download and cache template contents
and to throw an error when a template request fails.

BREAKING CHANGE

Angular will now throw a $compile minErr each a template fails to download
for ngView, directives and ngMessage template requests. This changes the former
behavior of silently ignoring failed HTTP requests--or when the template itself
is empty. Please ensure that all directive, ngView and ngMessage code now properly
addresses this scenario. NgInclude is uneffected from this change.