add "make_header_line" function to G/F/Bedpe/Header

When called on an empty header, the proper header line will be generated
and added (including the standard and any custom fields).

When called on a non-empty header, an exception is thrown.
update G/F/Breakdancer to be able to parse Squaredancer files

Squaredancer reports SVs in Breakdancer format with one notable
exception: the orientation fields. Breakdancer always reports + and -
counts (even when one is zero):

    10+0-

Squaredancer can report for only one strand, like so:
    10+

The parsing code now handles both possibilities.
Merge pull request #309 from tabbott/sv-fileformats

Sv fileformats