Upload and download routines

[kaetemi]

• String downloadText()
  • This would verify if content-type starts with text/
• downloadImage()
  • This would verify if content-type starts with image/
• downloadFile()
• uploadText(String text)
• (uploadImage?)
• uploadFile

Library should handle everything behind the scenes.

We also want a mechanism for pre-signing both uploads and downloads in the style of new Bucket.signed(string endpointUrl, string key, Map<String, String> signatureHeaders) as specified by issue #1.