Some small fixes

Here are a few fixes for DICOM.jl.

Reading sequences was pretty broken, since `sequence_item` didn't actually check how many bytes it read, and `sequence_parse` didn't include the 8 bytes preceding each item in calculating how many bytes had been read. The fix for `sequence_item` is the minimal possible change to the file, but relying on `position` is a bit kludgy.

DICOM.jl was also applying `integer` to the contents of VR `DS` (Decimal String); I changed it to apply `parsefloat`.