Corrected a bug in HairpinMapper.

Noticed a potential bug in HairpinMapper where, when mapping DNA,
G=T pairs would not be considered as G=U. Of course, this is
desirable for DNA, but as PySplicer's internal representation is
*always* DNA, I included a setting to consider G=T as equivalent
to G=U, which allows client objects/functions/applications to
determine whether they want to map hairpins including G=T base
pairs; this would be appropriate, for example, when designing DNA
that will be transcribed to RNA, as these base pairs, when
converted to G=U, become possible.