Sanitize file data before parsing with Jackson

[jhaber]

This takes the raw response from Mesos and decodes it with a UTF8 decoder that ignores invalid characters before passing the sanitized data to Jackson for parsing (which should now always succeed). Since these responses can be non-trivial in size I tried to avoid extra allocations as much as possible (the Response class exposes the data as a ByteBuffer which gets converted to a CharBuffer which backs the Reader passed to Jackson).

@tpetr @ssalinas