Change position of basepair label when resizing exon.

The base pair label drawn with the vertical line obstructs exon resizing. So we
position the base pair label towards the bottom of the track.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
Align vertical line exactly with cursor position.

Signed-off-by: Anurag Priyam <anurag08priyam@gmail.com>
snap to nucleotide boundaries when resizing exon

fixes #44

Signed-off-by: Harsh Gupta <gupta.harsh96@gmail.com>