Add computer name field to Record

[zrweger]

See https://msdn.microsoft.com/en-us/library/cc231412.aspx

The offset to the Computername is variable, pending where the SourceName terminates. I didn't want to recompute the offset just for the add_explicit_field call. I'm also a little more careful about checking that the terminator is actually word-aligned than the rest of the code, but alas.

[williballenthin]

another great addition, thanks!