Add ability to set fatfs label & retrieve serial number.

Currently, under ubuntu 14.04 anyways, the internal volume shows up as: /media/dhylands/4421-0003

This PR adds some functions which allow the volume label to be changed. If I put:
```python
import fatfs
fatfs.label('/flash', 'internal')
```
into boot.py, then the volume will instead show up as /media/dhylands/INTERNAL

Incidently, you can now retrieve the serial numer:
```python
>>> fatfs.serial('/flash')
1143013379
>>> hex(fatfs.serial('/flash'))
'0x44210003'
```
and we can see that with no label, the serial number formatted as hex is what's used.