(PUP-2972) Remove += and -=

These operators are being removed because users find them confusing.
There is no loss of power in the language, however, because they were
just shortcuts for . = . + . and . = . - .
Merge pull request #16 from zaphod42/issue/master/pup-2972-remove-plussignment

(PUP-2972) Remove += and -=