Fix byte handling to work for both Py2 and Py3
Add a basic DCD file test. There's no DCD file _reader_, so without MDTraj we
can't validate the that this object does anything more than simply write without
exceptions (i.e., we can't tell if the DCD file is actually _valid_)
Use tempfile to make a file.
Open as binary (needed for Python 3)
Make sure the DCD file handle is closed -- do it the right way in the test.

Make sure files are appropriately closed by PDBFile()
Merge pull request #722 from swails/fix_dcd

[WIP]Fix byte handling to work for both Py2 and Py3