Add distance tables from Dunham et al. (2011b)

New MRTs in the ancillary/ directory for distances derived in conjunction with
ammonia observations of molecular cloud structures.
Changed column label in ancillary/Dunham_2011b_table6.mrt

ApJ likes to have E_ and e_ represent the upper and lower error bars on a
measurement, when unequal.  However, in case-insensitive languages (like IDL)
this causes much pukage.  The columns have been renamed as p_ and m_ for
plus- and minus-error-bars.