Some improvements, major overhaul needed

- It turned out that PyTables was not a good solution as a backend:
    - It doesn't support concurrent writes
    - It gets easily corrupted when a program crashes while
      writing to it (and it happens all the time)
    - So managing a huge number of symbols (>10000) is really painful
- Next commit will change PyTables backend to pickled files
    - Will support concurrent writes as long as not two symbols are
      written EXACTLY same time (very unlikely to happen and can be
      managed)
    - If one file gets corrupted it can be easily replaced while
      keeping the other files intact
    - Pickled files will contain a dictionary with data and metadata
    - Saving and opening pickled files is 10-100x faster than
      hdf-files
    - Managing large amount of symbols should be much easier
- Other option would be something like Redis
    - Probably even quicker than pickled files
    - Supports concurrent I/O by using (de)multiplexing mechanism so
      it is not possible to corrupt the database by simultaneous
      writes
    - Even faster than pickled files because the data is held in
      RAM most of the time
    - Requires a separate process to run so that is not really handy