Merge pull request #550 from zizkovrb/mysql57-ci

Add MySQL 5.7 test to CI.
Improve the forced reconnect code in the init_command test.

Instead of using a timeout to trigger a connection close, explicitly kill the connection.
Instead of testing the return value of ping for reconnect, test that the connection ID changed.
Use AppVeyor for Windows testing
Skip EM specs on Windows
Update for MinGW-x64
Add support/libmysql.def
Generate interface library libmysql.a from MySQL Connector/C libmysql.lib and the pre-generated libmysql.def included here
Use env var RUBY_MYSQL2_LIBMYSQL_DLL to find libmysql.dll out of path
Turns out this is not a reliable test of whether libmysql.a was built correctly.
Vendor libmysql.dll by default on Windows. Use rpath only on Unix platforms.