Update BioSeq

Using @b_str in @nt_str, @aa_str and @dna2_str
This works fine for Julia 0.1 and 0.2, but without interpolation on Julia 0.2. This is updated on docs.
Faster percentGC using chunks and bit-wise operations on DNA2Seq.
Merge pull request #131 from diegozea/patch-2

Update BioSeq