More robust eol handling for Sequence::load_fasta

More robust eol handling for Sequence::load_fasta
The load_fasta code used getline() which only uses "native" end of line
conventions. Unfortunately for me OS X has "native" and "backwards
compatible" eol conventions (which aren't compatible). So this patch
will accept  CR, CR-LF, LF, and because I was lazy LF-CR.

I also unit tested that which required breaking load_fasta into a
function that  handles loading from a file name and one that works
from an iostream.