implement alphabet for sequence

implement alphabet for sequence
in order to fix ticket:144 when motifs were changed to be
sequences I needed someway of specifying what alphabet a particular
sequence is using.

This patch changes things so a sequences default sequence alphabet
is a "AGCTUN" AKA reduced nucleic alphabet.

Motifs however use the full IUPAC nucleic alphabet.

(Or at least I hope they do I tested the sequence class, but GUI
testing is harder).

Another change is that instead of building a fixed reverse complement
table, there's a function which creates the table and understands
that at least for reduced_rna_alphabet (currently the only specifically RNA
alphabet) the reverse of A should be a.

Sequence is no longer case sensitive, all of the comparisons
are now done in an insensitive manner.

Finally I found a bug in the motif scanning code that was due to a typo
from long before I inherited the code. Aparently that code branch in
motif_scan had never been executed before.