add phpmig.sets & add sets-key option

* support for migrations of database sharding
changed option '--sets-key' -> '--set'
Modification of the name of the file to be generated by the Generate command
changed phpmig.currentSet to phpmig.connection
changed how to check the phpmig.sets
Merge pull request #54 from nagodon/support-sharding

add phpmig.sets & add sets-key option