Makefile generalisation, Unicode support

Hi,

Here are some tweaks to firmata_test so it builds on Unicode wx2.8 builds, and also some changes to the Makefile so it's easier to override specific parameters (and also not rely on hard-coded wx-config paths).

I made these before I noticed the comments in #1 to the effect that this repo isn't canonical, and is just synced from pjrc.com periodically... I also hadn't noticed the "OSL" Unicode tarball on pjrc.com, which makes some of these same changes as well... Oops, my mistake going direct to github(!)

I think there are now 4 slightly divergent versions of the firmata_test code:
1. pjrc.com main tarball
2. firmata/firmata_test repo, currently identical to (1) but may fall behind in future. Requires ANSI wxwidgets build and has hardcoded paths in Makefile.
3. pjrc.com "OSL" tarball by Orion Lawler, can build on ANSI or Unicode wxwidgets and has hardcoded paths in Makefile.
4. freetronics/firmata_test_repo in this Pull Request, similar to (3) but also adds some tweaks to the Makefile so can fallback to default wx-config path if the hardcoded paths aren't present.

I have a bit of a bold proposal, apologies for popping up out of the blue and making this:

@PaulStoffregen, how would you feel about making firmata/firmata_test the canonical repo for firmata_test and working from here? The Unicode changes in this PR should be fully ANSI-compatible, so building the same code against ANSI wxwidgets 2.x should work. The Makefile changes should also be backwards compatible, I kept the hardcoded wx-config paths and just added a fallback to $PATH if they aren't found (this is untested on Windows, though).

If that isn't desirable, I think a good second best would be to mark this repo clearly as "non-canonical/unsupported/out-of-date" on the description and README and link to the same tarballs as firmata.org, so it's clear which version is canonical. I can submit a PR for this if that helps.

However I would prefer the first proposal, if Paul doesn't mind. It'd be nice to have a common place to find (and contribute to) this helpful code. :)

Thanks everyone for your work on firmata and this test tool, btw.