Add MySQL 5.7 test to CI.
Merge pull request #550 from zizkovrb/mysql57-ci

Add MySQL 5.7 test to CI.
Connector/C only ships Visual C++ libraries, which MingW 32 binutils can link, but MingW 64 cannot.
The workaround is an incantation of gendef and dlltool. Thanks to @phasis68 for help with this.
Update for MinGW-x64
Use AppVeyor for Windows testing
Reorg vendor:mysql
Add devkit task on Windows
more appveyor
compile vendor
compile more vendor
More places for vendor/libmysql.dll
fxiup
todo mysql
fixup extconf.rb
Can't find unix find
Skip EM specs on Windows
Add reimp tool for Windows
Build with reimp.exe / libmysql.dll.a as the interface library
Ping may return true if reconnect succeeded, so check that the connection id changed
CREATE USER before GRANT in the numeric database name test

MySQL 5.6 appears to be stricter that the user must exist first.