ENH+TST: Minor improvements for looking up private meta data

The find_private_section function will now ignore leading/trailing
whitespace in the private creator value. Added a function
find_private_element to return a specific private element.
BF: Fix import for six package
ENH: Improve find_private_element function and use it in csareader

Use the existing (and clearer) terminology in find_private_element
(i.e. use elem_offset instead of elem_least_sig).

Use the new find_private_element in csareader.get_csa_header. No
longer requires the (0x29, 0x10) tag to exist, after all a
different private reservation could be used. This change required
some updates to test_dicomwrappers so that any fake data actually
uses dicom.dataset.Dataset types instead of plain dicts.