allow StringStoresCompatibilityLevel to apply to mining structures by furmangg

As described here:
http://technet.microsoft.com/en-us/library/gg471589(SQL.110).aspx

AS2012 allows you to set the StringStoresCompatibilityLevel=110 property on dimensions or (distinct count) partitions.

AS2012 does not allow you to set this property on mining structures, and some customers who are building very large mining structures are running into this 4GB string store limit. Please add that property to mining structures.

As an alternative, add a database level property which sets the default string store compatibility level for the whole database, but can be overridden per object as desired.