pytables selection enhancements (to close GH #1966)
in pandas/io/pytables.py
1. added **str** (to do **repr**)
2. row removal in tables is much faster if rows are consecutive
3. added Term class, refactored Selection (this is backdwards compatible)
    Term is a concise way of specifying conditions for queries, e.g.
   
   ```
   Term(dict(field = 'index', op = '>', value = '20121114'))
   Term('index', '20121114')
   Term('index', '>', '20121114')
   Term('index', ['20121114','20121114'])
   Term('index', datetime(2012,11,14))
   Term('index>20121114')
   ```
   
    updated tests for same
   
   this should close GH #1996

also potentially a starting point to do things like this:
  df['df>0']...e.g allowing simple string expressions in criteria (Term is indpendent; I could move to a separete module.... - just a thought)
