Add linkedin.people.invite API support

Sorry about that. This code was written quickly at a Hackathon a few months back. I missed this when I gave it a skip over before making pull request. I'll remove it once I get a chance. Thanks. 

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On Wed, Nov 5, 2014 at 2:48 AM, Hamza Waqas <notifications@github.com>
wrote:

>> +                    "person": {
>> +                        "_path": "/people/email=glavin.wiechert@gmail.com",
>> +                        "first-name": "Glavin",
>> +                        "last-name": "Wiechert"
>> +                    }
>> +                }]
>> +            },
>> +            "subject": "Invitation to connect.",
>> +            "body": "Say yes!",
>> +            "item-content": {
>> +                "invitation-request": {
>> +                    "connect-type": "friend"
>> +                }
>> +            }
>> +        }, function(err, data) {
>> +            console.log(err, data);
> I'm little confused if you need to put `console.log` or `done` over here?  
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> Reply to this email directly or view it on GitHub:
> https://github.com/ArkeologeN/node-linkedin/pull/24/files#r19857367