Add Lib function to obtain abs-path for filename

This library function will be used in Crawler::lookDir to ensure that
the path added to the list of directories and the list of files is
absolute
Add Crawler::lookDir() helper func

The Crawler's main task involves examining a subtree directory structure
to find files of a certain type.

The first step in this is writing code to allow Crawler objects to look
at the objects in a single directory.  This added helper function does
just this: it takes a directory name, and two by-ref std::vectors (one
to fill w/ filenames, one to fill w/ dirnames).  The function then
looks at each object in the directory and places it in one std::vec
or the other (or neither, in the case of a special file, soft link, etc)