Comment lines at the beginning of BED files are incorrectly interpreted as group labels

[dpryan79]

If a BED file starts with a "#", that line is interpreted as a group label and computeMatrix will fail, since the number of labels won't match the number of groups.

[dpryan79]

Since I'm fixing this anyway, I'll also fix the following cases:

1. "#" lines in the middle of a file with no associated entries cause an error
2. Having label (#) lines in the middle, but then not at the end of a file, causes an error

These are easy enough to fix in heatmapper.py