Correct a typo.
Fix a typo in a comment.
Add counting of number of entries in skiplist index.
Export element count for skiplist indices.
Add the option to return numUsed for each index in getIndexes.

This is currently implemented only for skip list indices and hash
indices, but not for full text, geo or bitarray, since these do not
export or even count this number.
Merge branch 'devel' of ssh://github.com/triAGENS/ArangoDB into devel