Dump retrieval/caching times in /api/all

Node.RetrieveTime is stored as an int64 in both the database and Node
data structure, so it must be converted using time.Unix() before it can
be used as a time.Time type.
Use os/signal to listen for OS signals

This includes a small piece of code to listen for SIGHUP and reload the
configuration file when that happens. Thanks to @inhies for an example
code snippet.

Close #17