Use parenthesis for method definitions
Omit unneeded checks in validation.

There is nothing that says container_no should be a String
in the specs. If it's not a String (but maybe an unknown
Container object), we can request the `to_s` value and still
validate a container number.

Also, the regular expression, together with the `to_s` will
handle the nil and empty string cases.
Clean-up validation method.

 * Get checksum and number in a clean way.
 * Clean up checksum comparison
Refactor checksum calculation

 * Use multi assignment to create arrays the Ruby way
 * Clean-up formatting of char -> digit conversion
Merge pull request #3 from kabisaict/code-refactor

Some code refactoring