Default Extensible-config to not load locales

Does not work correctly for Ext < 4.1 so reverted to not loading locales by default. Still works fine for Ext 4.1+ but must be manually enabled.
Comment to clarify intended use for Extensible-config
Check in diffDays fix from previous commit

The original fix is in Extensible.js, this is just the compiled version
Fix null comparison bug

typeof null === 'object' is true, so switch to Ext.isObject instead to avoid that when byDay is null
Reverted Ext version checking for alternate classname dependencies

After reviewing and re-testing, it seems that the alternateClassNames as they were prior to Ext 4.1 are working as expected now under both Ext 4.1+ and 4.0.7.  At some point these were broken, but they seem OK for all supported versions of Ext now. Since those checks were causing issues with Cmd compatibility, and appear to be no longer needed, they've been removed.
Pass 'extjs' to all Ext.getVersion() calls

Works the same either way by default, but if application code registers a custom package name, the last-registered name will be used by default, messing up these calls. Passing 'extjs' explicitly avoids that issue.