Make a string class that tells us the file it came from.

Because we're using a StringIO object to produce the EPS pages when we
add man pages to the diary, the BeginDocument DSC comment is less
informative than it should be.  It has been telling us something like:

  %%BeginDocument: <StringIO.StringIO instance at 0x2396950>

So add a NamedStringIO(StringIO) class with a __str__() method that we
use to print more useful information, like this:

  %%BeginDocument: man git # page 1

Signed-off-by: Russell Steicke <russells@adelie.cx>