Refactor long __init__ method to call two sub-methods

This change looks bigger than it really is due to the indentation
level changing; see diff view ignoring whitespace.
More explicit error message
Test brittle relative paths in Bio.SeqIO.index_db (issue #267)
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)