Update indexing.py

It looks like duplicates are extraneous in this index. This change will allow storage of longer values and reduce storage/ iteration on lookup if the value to be indexed contains any duplication. 
Works for ContainsIndexer and IContainsIndexer index classes. 

The indexer can now index a string  made up of 500 of the same character in an index list 500 long now too hence the <= 500:
```
>>> value=''.join([str(0) for i in range(500)])
>>> len(value)
500
>>> length=len(value)
>>> len(list(set([value[i:j + 1] for i in xrange(length) for j in xrange(i, length)])))
500
>>> len([value[i:j + 1] for i in xrange(length) for j in xrange(i, length)])
125250
```
Merge pull request #91 from benvand/patch-2

Improve the ContainsIndexer by not storing duplicate strings