fix error message from badly formatted duration
#55: replace the unicode plus or minus sign
use +- instead of ~
Merge pull request #54 from benfitzpatrick/fix-interval-bad-parsing

Fix error message from badly formatted duration
#55: replace "+-" with "+"
Merge pull request #56 from benfitzpatrick/55.replace-unicode-plus-or-minus-sign

#55: replace unicode plus or minus sign with ascii-compatible form
Update CHANGES.md