Crypto fix and custom cache config

The encrypt/decrypt code in lib/encrypt.js is broken. It doesn't actually use a random IV per request as it's calling `createCipher(algo, key)` instead of the `createCipheriv(algo, key, iv)` variant.

Interestingly, this is also why using the `encryptor.encrypt(...)` function to encrypt the cached keys in [`getCacheKey(...)`](https://github.com/jclem/node-heroku-client/blob/68e2ef81d522af51c40fea0a93c6907702474c67/lib/request.js#L305) actually works. If it actually used a random IV then each encryption would give different results and you wouldn't be able to look up the same key again!

The attached patches fixes the encrypt/decrypt methods by switching over to [simple-encryptor](https://github.com/sehrope/node-simple-encryptor). It also changes the `getCacheKey(...)` to instead use the HMAC of the request key. That way it's irreversible, unique, and consistent.

It also adds a configure option to use a custom cache (ex: Redis!) instead of requiring Memcache/memjs. The cache API is the same as memjs. It must have a `get(key, cb(err, value))` and `set(key, value)` . As Memcache is now optional I think the dependency should also be removed from `package.json` though I haven't done that as part of this cache.

I've commented out some of the cache related tests so that everything passes. It should be easier to rewrite them now that a custom cache can be directly injected.