user-configurable expandableRowHeaderColDef

Related to issue #1990.

This allows the user to configure the expandableRowHeaderColDef by adding a $scope.gridOptions.expandableRowHeaderColDef to js files.
Please use var declaration for variables you are declaring. I could not find one for userInjectedExpandableRowHeaderColDef and finalExpandableRowHeaderColDef. Also code styling in the if block needs to be corrected.