Unit tests against #88

Sequences with only numeric FASTA ids are not properly retrieved using
blastdbcmd. While our hack fixes this, it is to be reported upstream.

Signed-off-by: Vivek Rai <vivekraiiitkgp@gmail.com>
Refactor and improve sequence_spec.rb. Minor improvement to Sequence class.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>