added `meminfo()` method; changed `info()` behavior.

Method added to get memory usage of DataFrame colums and `df.info()` changed to show total memory usage of the DataFrame.
Added/edited docstrings.
Retooled memory_usage implementation.

DataFrame `memory_usage` automatically displays units and `meminfo` returns memory usage of each column in bytes. This follows the discussion found here <https://github.com/pydata/pandas/issues/6852>.
Fixed typo.
Added `test_info_memory_usage`

Tests whether "memory usage: " is displayed in from `df.info(memory_usage=True)` when `memory_usage` keyword is asserted.
Added memory usage (Issue #6852) to enhancements list.
Added memory usage example.
Resolved conflicts in `doc/source/v0.14.1.txt`
Closes #7619. ENH: Dataframe memory usage.

Adds methods to view memory usage of a dataframe.