Offset offset

Fixes

• c193a4b This fixes the way offset is sent to Sourcekit, there's been some back and forth on this issue due to Sourcekit handling the offset inconsistently, but this has now been fixed since XCode 7.3

Enhancements

• d27b6af A separate temporary file is kept for each buffer, rather than re-using the same one. This should assist Sourcekit if it does or ever decides to index the given file.