first attempt at supporting an externally-created OleDbConnection
Changed the approach to let the library create and dispose of the
connection internally. Implemented as a constructor overload on
ExcelQueryFactory
Changed the approach to let the library create and dispose of the
connection internally. Implemented as a constructor overload on
ExcelQueryFactory
Use recommended approach for implementing IDisposable
Merge branch 'acorkery/master'

Conflicts:
	src/LinqToExcel/ExcelQueryFactory.cs
	src/LinqToExcel/Query/ExcelQueryArgs.cs
	src/LinqToExcel/Query/ExcelQueryConstructorArgs.cs
refactored UsePersistentConnection API