padding xpaths + generating repeat_count nodes + lowercasing column headers

Adding padding to generated xpaths
https://github.com/modilabs/pyxform/issues/74

Generating repeat count node
https://github.com/modilabs/pyxform/issues/75

Column headers are now case insensitive
https://github.com/modilabs/pyxform/issues/76

Changing unofficial settings instanceID and instanceName to instance_id and instance_name.