[Proposal] Added support for change column in Schema #4745 #895

This will allow doing the following:
```
Schema::table('tests', function($table) {
    $table->string('name')->nullable()->change();
});
```
New function `compileChange` enables changing columns in MySQL grammar. Similar implementation can be easily added to sqlite, sqlserver and postgres.
Please see doctrine/dbal library for details on comparator. In Laravel Schema's Grammar.php, function compileRenameColumn also depends on doctrine/dbal for a true DB agnostic approach.

The primary reason for using doctrine/dbal is inability of column rename and change features in sqlite. The compileChange function can be easily overridden in MySQLGrammar.php and other non-sqlite grammars like this:
```
	public function compileChange(Blueprint $blueprint, Fluent $command) {
		$table = $this->wrapTable($blueprint);
		$columns = $this->prefixArray('modify', $this->getColumns($blueprint, true));
		return 'alter table '.$table.' '.implode(', ', $columns);
	}
```