Record indexed filenames relative to the index - issue #267
Test changing folder (idx should be using abs paths here)
Supplement triple_sff.idx with new-style relative paths.

```
$ python3.4
Python 3.4.0 (default, Mar 17 2014, 11:50:28)
[GCC 4.4.7 20120313 (Red Hat 4.4.7-4)] on linux
Type help, copyright, credits or license for more information.
>>> from Bio import SeqIO
>>> d = SeqIO.index_db("triple_sff_rel_paths.idx", ["E3MFGYR02_no_manifest.sff", "greek.sff", "paired.sff"], "sff")
>>> len(d)
54
>>> quit()
```

New code stores relative paths relative to the index file, with flag
set in the meta data so as not to break old index files.
Test new indexes with relative paths work
Mention index path change in NEWS (issue #267)
Test some indexing relative path setups
Spot child-paths automatically for indexes
Directly check filenames actually recorded for index
Use Unix slashes when storing relative paths
Explicitly test relative vs abs paths in index
PEP8 tidying for SeqIO index work