updated akavache.nuspec
Merge pull request #40 from vevix/net-45-fix

updated akavache.nuspec
Fix Akavache for platforms that don't really matter
Version bump to 2.6.3 and other stuff
Add Xamarin.iOS and Xamarin.Android
GetAllKeys should return null if the key isn't found
Fix assembly name for Xamarin.Mac
Version Bump to 2.6.4
Stupid mistakes are Stupid

Closes #42