ReactiveCommand can set initial CanExecute state

ReactiveCommand's constructor can optionally set the initial CanExecute
state.  Originally, it defaulted to true, until your canExecute observer
told it otherwise.  It is of course possible to forced the state to
false before binding it to anything, but this removes that nescessity
Create some global constants for cache limits
Set the cache limits on a bunch of internal caches
Add an android manifest whatever just stop doing this constantly
Merge pull request #345 from sillyotter/reactive-command-initial-condition

ReactiveCommand can set initial CanExecute state
Use the correct constant for Xamarin.iOS
Merge pull request #346 from reactiveui/less-caching

Use less caching on mobile platforms
Don't instantiate all the items in the resolver if we only want the last one