add cache package

[Teelevision]

I extracted the cache from one of our services, because I thought this was already a pretty general solution that we could make use of in other places as well.

I'm a bit unsure about the naming. Because the package is already called cache I tried to avoid using cache, like cache.Cache and cache.RedisCache. So a struct that needs a cache could look like this:

type NameMap struct {
    Names cache.Strings
}

And initialisation like this:

nm.Names = cache.InMemory()
// or
nm.Names = cache.InRedis(redisClient, "prefix")

[Teelevision]

I added a test suite.

[daemonfire300]

Not sure if two exposed errors are enough but I couldn't think of more errors that are not already handled by handling contexts errors in general. So:

LGTM.