v6.0.0-beta.1

Changes after v6.0.0-alpha.3:

• Feature: Add two methods to RestSynthesizer that returns Bytes instead of Vec<u8>.
• Upgrade openssl dependency (Solves security alert #77)
• Add two examples for the rust crate
  • 01-synthesize-txt-files.rs: Synthesize speech from *.txt files in a directory.
  • 02-rssl.rs: RSSL, Read-Synthesize-Speak-Loop (Something similar to a REPL). Read text from stdin line by line, synthesize speech and play it.
• Internal refactor