MySQL optional OpenSSL dependency

[TobiasDeBruijn]

Refinery depends on MySQL when the mysql feature is enabled, However there is no way to specify the features yet. My PR to rust-mysql-simple adds a feature that allows end users to remove OpenSSL from their dependency tree.

This PR adds a feature mysql-notls to refinery which allows users to explicitly disable TLS support in MySQL, and thus OpenSSL. It does this by:

• Adding default-features = false to mysql in refinery_core
• Adding feature mysql-nativetls to refinery_core which enables the feature mysql/native-tls
• Adding feature mysql-notls to refinery, which adds the mysql dependency in refinery_core
• Modifes feature mysql in refinery to include refinery_core/mysql-nativelts
  This approach was chosen to remove backwards compatibility issues for users relying on refinery. Those that rely on refinery_core directly will however need to enable the refinery_core/myslq-nativetls feature to keep native TLS support.

This PR should not be merged yet, as the PR linked above has not yet been shipped in a version of rust-mysql-simple.

Any feedback is more than welcome :"D

[TobiasDeBruijn]

New PR as #202