fix($httpBackend): don't send empty string bodies

The `XMLHttpRequest.send` spec defines different semantics for `null`
than for an empty String: an empty String should be sent with a
`Content-Type` of `text/plain`, whereas `null` should have no
`Content-Type` header set.

Closes #2149
fix($animator): avoid completing the animation asynchronously unless CSS transtiions/animations are present

Closes #4023
Closes #3940
chore(grunt): switch to the new //# sourceMappingURL pragma

All browsers except from Chrome implemented both the old
"//@ sourceMappingURL" and the new "//# sourceMappingURL" pragmas
in the same version so the only reason to keep the old one was Chrome.
However, Chrome 29, i.e. current stable version already supports
the new pragma so there's no need to wait any longer.
fix($httpBackend): set headers with falsy values

This is a breaking change. To migrate to the new behavior,
delete or set headers to `undefined` to avoid having them sent.
To restore the old behavior, override `$httpBackendProvider`
with the old implementation.

Closes #2984
docs(ngRoute): add angularEvent param to $routeChangeStart event

Adds missing implied first argument, `angularEvent`, to match
the rest of the `$routeChange` event documentation.
fix($compile): allow interpolations for non-event handlers attrs

Fix wrong behaviour that didn't allow 'data-on' and 'on' element attributes
to be interpolated by $compile. The regex now accepts any string beginning
with 'on' and with at least one more English letter.
feat($compile): support tel: links in a[href]

Allow `tel:` links so that click-to-call works in mobile browsers
feat($location): add support for state and title in pushState

Adds $location pushState & replaceState methods acting as proxies to history
pushState & replaceState methods. This allows using pushState to change state
and title as well as URL. Note that these methods are not compatible with
browsers not supporting the HTML5 History API, namely IE<10.