Add support for .NET 4.5

[yukozh]

I am working on using MySqlConnector replace the MySql.Data for Azure database for MySQL. Since our project is still in net45, not net451, could you add support for net45?

[bgrainger]

It looks like we changed it to net451 in #154 at your request. However, I don't see any hard dependencies (in our code) on net451 so I think we could change it back to net45.

[bgrainger]

Fixed in 0.23.0.