Quantize

Following the discussion at https://groups.google.com/forum/#!topic/pytables-dev/-drMFDFrv-Y, I started adding a function to truncate floating point data before writing to disk, to a specified precision. This can significantly improve the performance of compression.

Open points:

* example usage in Array docstring
* some more documentation with examples and compression benefits, to go 
  WHERE?
* packing and unpacking of the filter with the added 
  least_significant_digit. I need some help here.
* implement this for Tables. So far, I've only ever used arrays, so I'm 
  not sure where to do this. For arrays, I used the Array.__setitem__ 
  method. Where's the equivalent point for Tables?
* The array on disk should get an attribute "least_significant_digit". 
  I still need to find where I can set this.