added history.back method

Added this so that the router is not directly relying on the
window.history object
Merge pull request #232 from tommck/upstream

added history.back method
Added system.isPromise and enabled promise conversion in observable.

The observable plugin will now detect promise properties, wire
callbacks, replace them with observables and update their values when
the async task completes.
Making the router.reset remove all event handlers.
Fixing the minimum required jquery version.
Fixed "pack" and "package" pointing to non-existent "package" instead of "pack".
Merge pull request #233 from thinkloop/master

Fixed "pack" and "package" pointing to non-existent "package"
Modal dialog models now receive detached callback.

Removed api docs from source.
Ensuring that generated api docs are not included in source.
Optional param replaceRoute added to mapUnknownRoutes...

...this allows the developer to easily specify a module *and* a route to
replace the url with when an unknown route is encountered.
Ensuring no bom on dist lib.
Merge branch 'master' of github.com:BlueSpire/Durandal