Field size calculation not to spec

[JMarlin]

This should resolve #11 and issues like it; The major underlying problem is that the calculations beginning around line 341 are making the assumption that the header will always be the size of the code-defined header structure, however per the spec the header can have a completely arbitrary size.

The main correction needed here would be to calculate the proper number of field entries in the header by reading each field struct starting at offset 32 and increment a counter accordingly until the first char in the field name is equal to the field record terminator (0x0D)

This is per the definitions described here:
https://msdn.microsoft.com/en-us/library/aa975386%28v=vs.71%29.aspx

I think I'll fork and fix this over the weekend.