Fixed array to string conversion error

Hey,

I ran into some trouble with running the mink-extra contexts behat statements for tables.

I put together a solution that's working for me and I thought you might want to merge in my fix.

I've never contribute here before so if you need additions to this PR just let me know.

Thanks!
Matthew Boatman

- Fixed array to string conversion error in:
    TableContext::assertTableRowContains
    and TableContext::assertTableColumnForTableRowContain
- Added: TableContext::printTable