update HISTORY.rst with recent changes
implement sandbox --datadir option

- Adds --datadir top level option for allowing a user to specify
  where the sandbox datadir should be located, otherwise defaults
  to ./data/ relative to the sandbox directory as before.
- Requested datadir will be check to verify either:
  1) datadir does not already exist (and will be created + bootstrapped)
  2) datadir exists, but is empty (and will be bootstrapped)
  3) datadir exists, is not-empty but not in use by mysql
  4) datadir otherwise looks like a normal mysql datadir
     (currently ./ib_logfile0 exists or ./mysql/user.frm exists)
- The -D short option previously used for --data-source is now
  a short option for --datadir.  --data-source now has a '-s'
  short option.  This is an incompatible change.

Resolves issue #68