More robust naming

The provisional names generated for MPC files should now be unique even if the application is started and stopped.  The calculation is as follows:
```
base52(exposure_number) + base62(int(reading.x + reading.y))
```

Base 52 is used on the exposure number to make sure we start with a letter (as per the spec).  If the name is less than 7 characters it is padded with 0's on the right.

Fixes #125