General restyling - Removed deprecated statements and updated dependencies

[veeso]

Hello, I hope you'll accept this pull request, since this library is really important for Rust FTP support. Really, there's nothing decent out there for FTP in Rust.

These changes solve deprecation warnings and updates all the crates to the latest version. In addition I replaced openssl with native_tls to make it easier to run on Windows and MacOs machine.

Changes

• Replaced OpenSSL with Native-tls
• Upgraded dependencies:
  • lazy_static 1.4.0
  • regex 1.4.2
  • chrono 0.4.19
• Fixed deprecated statements
• Added get_welcome_msg method
• Solved these issues:
  • Outdated openssl (0.9) does not allow to compile at systems using OpenSSL 1.1 #91
  • Upgrade openssl to 0.10. or replace it with rusttls #84

I tested the commit and everything is fine.

EDIT (2020-12-02)

I also merged what you already did on branch stable-updates, which sounded good to me.

EDIT (2020-12-03)

I reported the patches from async-rust-ftp (which is a library derived from this one).
Eventually I replaced openssl with native_tls, which makes it easy to run on MacOS and Windows.

---

For those who read this merge request and are looking for a way to apply these changes before the new published version, just add this to Cargo.toml

# ...
[dependencies]
# ...
ftp = { git = "https://github.com/ChristianVisintin/rust-ftp" , features = ["secure"] }
# ...

[mattnenterprise]

@ChristianVisintin Thanks for making these updates. I plan on actively maintaining the project from here on out. Seeing as your an active user of this library any feedback on it would be nice.