Add a simple GYP file

I'm starting to play with libsphia with node, so a GYP file will be very helpful.  I've only briefly played with GYP, so I'm sure there's quite a bit missing here.  Furthermore, I've only tested on OSX.

I'll try to keep this updated as we progress.

The current GYP file in action:

```
$ gyp --depth=. --generator-output output -f make
$ make -C output/
  CC(target) out/Default/obj.target/sphia/src/clear.o
  CC(target) out/Default/obj.target/sphia/src/count.o
  [ . . . ]
  CC(target) out/Default/obj.target/sphia-test/test/set.o
  CC(target) out/Default/obj.target/sphia-test/test.o
  LINK(target) out/Default/sphia-test
  LINK(target) out/Default/sphia-test: Finished
$ ./output/out/Default/sphia-test

running 'sphia_new' test(s)
-------------------------
  [ . . . ]
-------------------------
  libsphia test results
-------------------------

  passed: (8)
  failed: (0)
```