Add missing dependency to README file

The required Python driver to MongoDB, pymongo, was not mentioned is
"Requirements" section of the README file.  I fixed it.
Add transform converter

I added a small utility tool allowing to convert transforms from their
"raw" format to a proper database collection.
Merge branch 'queue' from the main repository

Merging changes from the main repository.
Correct import in transform converter

Fixed incorrect import of JSON schema in transfrom converter.
Fix iterator

In the loop over transforms, iterator was going through dictionary keys
(transforms' SMARTS) only instead of its items (both SMARTS and
reactions).  I fixed it.
Improve elimination of invalid transforms

Try-except clause eliminating invalid SMARTS were not catching
ValueError exception raised when a transform's SMARTS is incorrect.  I
added it to a list of exception to be caught.
Update README file

I updated location of the file describing transform database entry.