custom field creation API/BAO: support passing name

if using the custom_field API to create custom fields, you cannot set the field name value. when passing that value to the params array, it is ignored and reconstructed using the label param.

since the field name is a fixed value once the field is created, whereas the label may be altered via the UI, the ability to set and later reference the name is very useful to developers. we should alter the BAO to support passing the name param.