Issue740: introduce bucket plugin to organize files in bucket directories

see issue #740 for the reasoning

- new plugin `bucket`that provides a function `bucket{$field}` to use for path formatting
- match a year/any textual field with a year/alphabet range defined a priori in the plugin config 
- docs + unit tests

**TODO**

- [x]  `BucketError` raised to signal invalid bucket formats should probably be raised more often
- [ ] not sure of the *range of ranges* to precompute : I generate all the ranges between 1900 and 2014. But because of classical music should we go back as far as 1600 or 0, etc ??
- optional: `normalize_year` code could be more elegant (modulo ops :/ )