Add .info_mem() method to pandas data objects

[ghost]

Series, frames and panels are all built on numpy arrays, for which
we can calcuate memory footprint.
For python objects as well we can determine footprint based on platform,
python ver/bits and object type.

We can use thse to generate a report of amount of memory used and where.
That would be useful and not difficult.

[jreback]

superseded by #6852