Inserting DATE-OBS keyword

This branch adds the function `insert_dateobs()` to `run_astrodrizzle.py`. AstroDrizzle has the unfortunate habit of removing the DATE-OBS keyword from WFC3 and ACS files (curiously, it doesn't do this for WFPC2), and this function reinserts the keyword using the header information from the original input file.

It also has `run_astrodrizzle()` remove unwanted `_d2im.fits` AstroDrizzle WFPC2 outputs, keeping our output archive nice and clean. 

Ticket #154 is the original ticket for this branch.