Edited Issue: CodeFirst: Each call to Map/MapToStoredProcedures overrides the previous ones, even if they were mutually exclusive [934]

When making multiple calls to MapToStoredProcedures, we override the previous map stored proc settings. This is a bit confusing. For instance:

``` C#
modelBuilder
.Entity<Blog>()
.MapToStoredProcedures(s =>
s.Update(u => u.HasName("modify_blog")));

modelBuilder
.Entity<Blog>()
.MapToStoredProcedures(s =>
s.Delete(d => d.HasName("delete_blog")));

modelBuilder
.Entity<Blog>()
.MapToStoredProcedures(s =>
s.Insert(i => i.HasName("insert_blog")));
```

will only change name for the Insert stored proc. Other two will be ignored. We should be keeping the previous settings as well.