Address an import parsing problem.

I've got a binary here which pefile parses imports correctly but YARA
does not. It appears that when parsing the Import Directory if
OriginalFirstThunk is zero you can use FirstThunk.
Merge pull request #197 from wxsBSD/import_fix

Address an import parsing problem.
Fix false positive in "fullword" matches when string is declared both "ascii" and "wide"

Also improved detection of strings fitting in atoms and removed redundant code for "fullword" handling.