getFreeSpace & getTotalSpace methods in data persistence layer

[nichhk]

Required by #57

[jfking]

@nichhk We'll try to get this done tonight

[prb2]

@jfking Any update on this? I'm about to start working on #60 but I'd like to wait till this is in develop before I branch off

[jfking]

Already talked on Slack, but this will be in as soon as #63 merges