Add the ability to specify a mysqli socket path.

Update the MySQL connection to support a non-default socket path. When using an application like MAMP, the socket path is not the default socket path (unix:///var/mysql/mysql.sock). Instead, the proper socket path is available via MAMP's Web Start (typically, /Applications/MAMP/tmp/mysql/mysql.sock)
Merge pull request #88 from barchard/master

Add the ability to specify a mysqli socket path.