pytables selection enhancements (to close GH #1966)

[jreback]

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 PyTables enhancements for selection #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)

[jreback]

see #2264