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 $compile minErr each a template fails to download
for ngView, directives and ngMessage template requests. This changes the former
behavior of silently ignoring failed HTTP requests--or when the template itself
is empty. Please ensure that all directive, ngView and ngMessage code now properly
addresses this scenario. NgInclude is uneffected from this change.
fix($animate): ensure guarded animations consider AJAX requests upon bootstrap

Prior to this fix when an Angular application is bootstrapped it would only
place an animation guard to prevent animations from running when the application
starts for the first two digest cycles. However, if any controllers or directives,
that are executed upon boostrap, trigger any remote code to be downloaded (via $http)
then the guard does not put that into consideration. This fix now properly addresses
that circumstance and removes the guard once all outbound HTTP requests are complete
when an Angular application is bootstrapped.

Closes #8275
Closes #5262