Added limited seektable support (upto UInt32 sample point/offset)

I am only looking at the least significant 32 bits of sample number and offset data this is more than sufficient for the longest flac file I have (~50 mins 2-channel 16-bit 44.1k which uses about 7.5% of the UInt32 space for the largest offset). It can certainly be improved by storing sample numbers and offsets as [Unit32, Unit32] (or something like closure's long implementation), but would require additional overriding of the searchTimestamp and seek functions (possibly more?).