Replace Map with Interface when retrieving `ColumnFamily` instances.

Try to envision a time when a metric may be stored in any of a number of different column families.  This change will make it easier to amend how we look up CFs in the future.
rename map
Merge branch 'master' into utility_method_for_getting_CF
Merge pull request #135 from gdusbabek/utility_method_for_getting_CF

Simpler interface for retrieving ColumnFamily instances (blocks statds integration)