DefaultDownloader, Clean Up, Auto-Detect, Moarrrr

- Wrote DefaultDownloader class using the basic Java HttpURLConnection.
Ugh I hate it.
- Refactored OkHttpDownloader variables to be generic
- Cleaned up old OkHttpClient code in PkRSS.java
- PkRSS now logs the logging upon logging (Yo dawg)
- Base Downloader now requires clearCache() method
- Utils class for general reusable code
- Now detects if OkHttp is present and uses it by default if it is (I
used the code block found in Picasso for this)