Allow creation of a DbMigrator with an explicit DbContext instance

[ajcvickers]

This prevents migrations having to attempt to create connections from connection strings.

Fixes #522

[CZEMacLeod]

Really useful change - I can finally get rid of the following code from my MigrationDBInitializer class... 😃

Private Shared ReadOnly _createNewMigrator As Reflection.ConstructorInfo
Shared Sub New()
    _createNewMigrator = GetType(Entity.Migrations.DbMigrator).GetConstructor(Reflection.BindingFlags.NonPublic Or Reflection.BindingFlags.Instance,
                                                                            Nothing,
                                                                            {GetType(Entity.Migrations.DbMigrationsConfiguration),
                                                                            GetType(DbContext)},
                                                                            Nothing)
End Sub
Protected Shared Function GetNewMigrator(ByVal configuration As Entity.Migrations.DbMigrationsConfiguration,
                                            ByVal usersContext As DbContext) As Entity.Migrations.DbMigrator
    Return _createNewMigrator.Invoke({configuration, usersContext})
End Function