Add a method to rename fields easily

[stokescomp]

I made a method for renaming a field.
I will add a pull request for it.
It will make it possible to do this:
$builder = Builder::fromFile('file.dbf')->renameField("name", "newname")->build('file2.dbf');

Instead of this:

$builder = Builder::fromFile('file.dbf');
$builder->getField("name")->setName("newname");
$builder->build('file2.dbf');

[majkel89]

Very nice. Just file the PR