follow perl recommendations:

* always use warnings
* use parent instead of base
* declare package variables with our, do not use vars
Bio::SeqIO::fasta - implement block method for pretty fasta

* size of a block is the length of sequence before introducing a space. Nice
aesthetic effect which eases finding position in a sequence.
* as side effect, implemented a base to have more fasta specific parameters avoiding
repeated code.
Merge pull request #51 from carandraug/pretty-fasta

Implement pretty fasta