[0.2.0] match php's key padding, allow key lengths 16, 24, 32

Matches some functionality from mightydes/node-rijndael@b0f64fc, but
implements the padding in a faster manner by preallocating the buffer
and filling it with Buffer#fill. Improves

Version bump to 0.2.0 because this is an API change.

Pull request #6 will require a rebase.

Signed-off-by: Eli Skeggs <eskeggs@globesherpa.com>
Update README for buffer/string compatibility

Signed-off-by: Eli Skeggs <eskeggs@globesherpa.com>
Merge with 0.2.0