Added option to replace previous browser history entry on transition

(splitted from this pull request https://github.com/angular-ui/ui-router/pull/389 so as to be a distinct change)

Added optional "replacePreviousHistoryEntry" parameter to state navigation options:
- if set to 'true' (default 'false') then the transition/navigation will replace the previous browser history entry
- if not set transition/navigation works as until now

A use-case for this would be when you have a state for an "add" screen and on "save" on that screen you navigate to a "details" state. However, if then the user goes back in the browser he'll end up on the "add" screen again, which might not be ideal: using this parameter as 'true' will cause the back navigation to actually go to the previous page (i.e. before the "add" screen).