BUG: issue in HDFStore with too many selectors in a where

- this is very hard to reproduce, and can give a ValueError or just crash (as somewhere in numexpr or hdf5 it has trouble   parsing the expression, and can run out of memory), easiest solution is just to limit the number of selectors

  just to be clear, you really have to do something like this:

  select.('df',[ Term('index',['A','B','C'......]) ])
  where the term has more than 31 selectors (and less than 61) to produce this error and only sometimes
  it triggers.

  This doesn't change functionaility at all, as if there are more than the specified number of selectors, it just
  does a filter (eg. bring in the whole table and then just reindex), and this is really only an issue
  if you actually try to specifiy many vaues that the index can be (which usually isn't a string anyhow)

  but I did hit it! so its a 'buglet'

  heres the issue in PyTables (but it is actually somewhere in numexpr)
  http://sourceforge.net/mailarchive/message.php?msg_id=30390757

- added cleanup code to force removal of output files if the testing is interrupted by ctrl-c
- added dotted (attribute) acces to stores, e.g. store.df == store['df']