Code dump!

This will diff a simple VCD file that I got from the output of a
Chisel counter test.  It will pass VCD files that are exactly the
same, and fail VCD files that are different.
Rename test cases

Now I've got a systematic name for test cases:

    test/vcddiff/D-NsigMdiff.bash

where

  * D is the design name
  * N is the signal difference
  * M is the value difference

Essentially there's two paremeters that I'm testing for, and I'm
sweeping all of them.
Add signal name test cases

This ensures that vcddiff is parsing the full signal names correctly,
not just the short names.
Improve the help text

You can now supply "--help" (in addition to no arguments) to see a
help text.
Add --version

This prints the version number.
List commandline arguments in help text

Hopefully this is sufficient documentation for users.
Add a README

This at least tells users how to build it (ie, use pconfigure) and how
to run it (pass --help for information).
Add a two-diff test

This has two changes, one at the start and another later on.
Run all tests under valgrind

I figure it's probably good to test everything under valgrind, just in
case.  It's not super useful as I'm not testing for memory leaks (I
know I have a bunch), but at least it'll catch some errors...

The tests still run under non-valgrind just to be safe.
Remove an unused test

I was originally going to enforce that the cycle number of the two VCD
files being diffed was the same, but I'm not sure that inside the VCD
class is the correct place to do that.  Certainly one could envision
this being used to diff two files which have slightly different cycle
numbers (a fixed offset, for instance), so I guess it's sane...