io: add chdir

Adding the chdir function, that will change the current directory of any
running chapel program.
Merge pull request #323 from fsouza/chdir

io: add chdir [merging for fsouza]

Adding the chdir function, that will change the current directory of any
running chapel program.

This includes two tests: one that tries to chdir to a directory that doesn't
exist, and other that calls chdir, and then writes a file to the current
directory (a new one).