Allow spreadsheet source data on a row different that 1

Problem/Motivation

In the real world the header row doesn't necessary start in row 1. There are spreadsheets having the table header after a title or other content. Right now MigrateSourceSpreadsheet assumes that the header row is the first row in the spreadsheet. The only way to workaround this is to extend the class and fork a lot of code.

Proposed resolution

Introduce a first_row the $options array that is passed to constructor, defaulting to 1.

Remaining tasks

None.

User interface changes

None.

API changes

Migrations can specify a different starting row by passing the value in the $options array as first_row key.

class MyMigration extends Migration {
  ...
  $this->source = new MigrateSourceSpreadsheet(
    'foo.xls',
    'Sheet1',
    array('bar', 'baz', 'fooo'),
    array('first_row' => 4) // The header is on the 4th row.
  );
  ...
}

Data model changes

The content of $options array accepts an integer keyed as first_row.