TST: Add tests for the Rfam related code
TST: Add tests for utility functions in Rfam

The following functions were untested in skbio but were tested in
pycogent, the test-cases were ported over and a few minor changes were
made to take advantage of TestCase object:
    - is_rfam_header_line
    - is_rfam_structure_line
    - is_rfam_seq_line

Five failures remain.