sysinfo memory update and bugfix

This change should fix #110 and also:

* Drop the functions relating to finding information about the swap since we don't capture this information on the master anymore.
* Cast to int directly in memory.py so we're doing this in one place instead of everywhere we call into memory.py (we're casting to into because that's all the DB will accept)
* Rename ram_free/ram_used -> free_ram/used_ram to better match the names in the config and database column names.