Use `dataWithBytes` instead of `memcpy`

Pursuant to discussion on pull https://github.com/ccgus/fmdb/pull/271, I am submitting new pull request that eliminates `memcpy` function call from `dataForColumnIndex` method, and ensures that it returns non-mutable `NSData`. Gus, certainly feel free to use or disregard as you see fit.

BTW, I reviewed not only the [`NSData` documentation](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/Foundation/Classes/NSData_Class/Reference/Reference.html) method, but also empirically validated that `dataWithBytes` truly copies the buffer, rather than using it _in situ._

No additional test cases needed, as `testBlobs` seems to cover the bases.