Add cache times to credentials.json

[ptlthg]

Adds configurable cache times (in seconds) to the credentials file.

While I don't really think the credentials file is a great place for this, there's not really a better spot and this functionality definitely needed to go somewhere.

I also added a cachetimes.js file which exports helper functions to check if a cache on a specific resource is expired. These functions are then used to make the cache checks in lib.js much easier to understand.

Default cache times might want to be changed, there were some very confusing checks that I replaced with this PR, so make sure to double-check those.

Right now they're defined in the credentials file like this:

cacheSeconds: {
  profiles: 60 * 5, // 5 minutes
  bingoProfile: 60 * 5, // 5 minutes
  museum: 60 * 30, // 30 minutes
  guild: 60 * 60 * 24, // 24 hours
},

[Shiiyu]

lgtm, yet to test, will do tomorrow