compare() function missing from exports

Node REPL fails to find compare() function:
```
> var jsonpatch = require('fast-json-patch');
undefined
> var objA = {user: {firstName: "Albert", lastName: "Einstein"}};
undefined
> var objB = {user: {firstName: "Albert", lastName: "Collins"}};
undefined
> var diff = jsonpatch.compare(objA, objB);
TypeError: Object #<Object> has no method 'compare'
    at repl:1:22
    at REPLServer.self.eval (repl.js:110:21)
    at repl.js:249:20
    at REPLServer.self.eval (repl.js:122:7)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)
```
Added export, fixed:
```
> var jsonpatch = require('fast-json-patch');
undefined
> var objA = {user: {firstName: "Albert", lastName: "Einstein"}};
undefined
> var objB = {user: {firstName: "Albert", lastName: "Collins"}};
undefined
> var diff = jsonpatch.compare(objA, objB);
undefined
> diff
[ { op: 'replace',
    path: '/user/lastName',
    value: 'Collins' } ]
> exit()
```