Always recalculate tickDecimals and tickSize.

Resolves #1000.  In Flot 0.7 we only calculated tickDecimals and
tickSize once, when creating the tickGenerator for the first time.  This
meant that calls to setupGrid failed to recalculate the values, as
reported in #860.  #861 fixed the problem by moving calculation into
tickGenerator, but this turned out to cause a new problem, since the
function doesn't run when an explicit ticks array is provided.

This commit solves both by always recalculating the values outside of
the tickGenerator function.  As far as I can tell the only reason it
wasn't done this way from the beginning was to avoid unnecessary work in
the case where tickGenerator is already provided in the options.  But
the extra work is negligible, and it's actually more consistent for the
properties to always be set.