Tweak memory usage

This is a candidate to go to 0.2.

If this PR passes, I'll bring those changes to master as well (sorry it should be the other way around, but it was useful that way for my tests with node 0.8...).

I have one last issue to solve, the shebang trick does not work:

```
./bin/hipache -c config.json
/usr/bin/env: node --expose-gc --always-compact: No such file or directory
```

Instead I tested with:

```
node --expose-gc --always-compact ./bin/hipache -c config.json
```

I'll keep digging into this...

Fixes #98