feat($location): add ability to opt-out of <base/> tag in html5Mode

This feature allows disabling Angular's requirement of using a <base/> tag
when using location in html5Mode, for applications that do not require
using $location in html5Mode in IE9. To accomplish this, a getter/setter method 
was added to $locationProvider called baseOptOut, which accepts a single boolean 
parameter. The method is semantically similar to html5Mode() (returns this if
called with bool param, otherwise returns current value).