Improve logging

This implements parts of #152.
Mainly the `IPFS_DEBUG` flag and the `ipfs log` command to change log levels of a running daemon. I chose add a explicit `GetEnvBool()` to the `util` pkg.
`Debug = true` should not be happening in here. It should probably be happening in `main()`: 

```Go
u.Debug = u.GetenvBool("IPFS_DEBUG")
```

Then, maybe let's keep the old logic here and make this:

```Go
if Debug {
   lvl = logging.DEBUG
}
```