Making asm.js an easier-to-use node module

Hi Dave,

*If you're already working on this or there's something else in the pipeline, or if this pull request is generally off the mark, please disregard.*

---

Currently `main` in `package.json` points to `lib/asm.js` which doesn't exist. I've created a `lib/asm.js` -- it just includes the basic stuff an asmjs-node-module consumer might want to access (at the moment: validate, types, ValidationError).

I also noticed the nodeunit tests were not succeeding so I've restored tables.js (which was missing [?]). Tests succeed now:

```bash
$ nodeunit tests/index.js

index
✔ testModuloIntish1
✔ testModuleIntish2
✔ testIntCoercionRequiresDouble1
✔ testIntCoercionRequiresDouble2
✔ testNot
✔ testParamTypes
✔ testAdd
✔ testImul
✔ testLoad
✔ testStore
✔ testCall1
✔ testCall2
✔ testVoid1
✔ testVoid2
✔ testEval1
✔ testEval2
✔ testEval3
```

Also changed `tests/asm.js` to `asmAssert.js` for clarity and to avoid ambiguity with the new `lib/asm.js`.

Please let me know if any further changes are required.