Changes to satisfy PerlCritic

lib/Crispr.pm - Don't modify $_ in map.
lib/Crispr/OffTarget.pm - Don't explicitly return undef.
lib/Crispr/PrimerDesign.pm - Use 3 argument form of open.
lib/Crispr/Target.pm - Don't explicitly return undef.
lib/Crispr/crRNA.pm -  Don't explicitly return undef.
Changes to dist.ini and .gitignore

dist.ini - Added code to not add cover_db/ and t/data/mock_genome* to dist.
.gitignore - Added .build and perltidy.LOG.
scripts - Altered Documentation.
New File: scripts/design_pcr_primers_for_illumina_screening.pl
Script to design Illumina sequencing primers for screening of crispr
guide RNAs or guide RNA pairs.