Remove email column from nodes_cached table
Add helper function for database caching

GetAllFromChildMaps() concurrently caches nodes from a list of
addresses. This is a huge step towards functional node caching and full
decentralization.

Ref #19
Set new map IDs properly and fix a typo

Previously, GetAllFromChildMap() would set new map IDs as
len(sourceToID)+1, but it should have just been len(sourceToID).
Add periodic child map caching

This commit includes a helper function, UpdateMapCache(), which clears
the current cache and calls GetAllFromChildMaps() with the argument
being the new configuration option, ChildMaps.

This means that nodes from child maps will cache properly, and will be
refreshed on every heartbeat. It is done very inefficiently at the
moment, and map IDs are not garuenteed to be correct, but it is very
close to usable.

Ref #19
Fix up map source IDs

This was the last minor issue required for functionality.

Close #19
Version 0.3