Fix Ascii IFD reading as TIFF6.0 spec

[TheZ3ro]

This PR fixes #187

The TIFF6 specification writes:

Value/Offset
To save time and space the Value Offset contains the Value instead of pointing to
the Value if and only if the Value fits into 4 bytes. If the Value is shorter than 4
bytes, it is left-justified within the 4-byte Value Offset, i.e., stored in the lower-
numbered bytes. Whether the Value fits within 4 bytes is determined by the Type
and Count of the field.

python-docx by default uses the Value/Offset field as a offset for ASCII IFD, but sometime the Value/Offset field could contain directly the Value (if it contains a string with 4 or less characters including the final NULL byte/string terminator)

This is the case for the 2 pictures linked in issue #187 (the Software ASCII field contains 8.3)

[TheZ3ro]

@scanny can you kindly merge this? Thanks!