add datasource option

To be able to connect to another datasource than the default one.
I’m not sure about the `-d` option, sounds like ‘delete’, any idea for
another short option for `datasource` ?
replace datasource with connection
add info for plugin and connection options in readme
Merge branch 'master' of https://github.com/cake17/cakephp-migrations into cake17-master

Conflicts:
	src/Console/Command/MigrationsShell.php
Merge branch 'cake17-master'