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.
```Go
	UsageLine: "log <name> <level> ",
	Short:     "switch logging levels of a running daemon",
	Long: `ipfs log <name> <level> - switch logging levels of a running daemon 

   <name> is a the subsystem logging identifier. Use * for all subsystems.
   <level> is one of: debug, info, notice, warning, error, critical

ipfs log is a utility command used to change the logging output of a running daemon.

`
```