add binwidth option

binwidth of 0 automatically resets to 0.1
change existing variable bin_width to binwidth to match bincount
modify bin calculation to include the max data value
max values and y-val of 0

include max data values (previously were left out of plot)
include max y values (previously were left off of y axis)
show y scale such that height of column in histogram corresponds to
  correct y value (previously corresponded to y-1)
no need to include 0 in ys. Rather, start ys at min of 1, and
  leave cols with y val of 0 empty.
Merge pull request #11 from mc818/max_values

max values and y-val of 0
Merge pull request #12 from mc818/binwidth

add binwidth option