Missing score represented by a dot "."

0 usually means that the score exists and is equal to 0.
Since we do not manage the scores (yet), a missing score should be
denoted with .
Better sequence identifier in the GTF file

Since coordinates are expressed in term of absolute positions on the
chromosome, the sequence identifier should be chrZZ.
Corrected stop codon frame (fix #39)

The frame must be computed separately for each feature.
The frames of the start codon and of the CDS are always the same, since
the start codon is part of the CDS.
The frame of the stop codon might differ if the stop codon is split over
two exons of if CDS has length that is not a multiple of three.
Merge branch 'master' of github.com:AlgoLab/PIntron into develop
Temporary.