Issue title: Error while running when running a test scan 
 Issue body: Hi,

I am trying to run a basic scan which returns an invalid uuid error. Please help 
The code :+1: 
scan = ness6rest.Scanner(url='<Nessus URL>', login='<username>', password='<password>', insecure=True)
scan.scan_add(targets='<Target IP address>')
scan.scan_run()

Attached a screenshot of the error
![err](https://user-images.githubusercontent.com/9638468/28980783-1cc3500a-791d-11e7-9d65-d86b8eeffe23.PNG)
 Comments: 
 Comment 0: The values of url, login, password and targets not included for security reasons.
 Comment 1: The same error, confirmed.

import ness6rest

scan = ness6rest.Scanner(url="url",
                         login="abc", password="cba",
                         api_akey='api_k',
                         api_skey='api_s',
                         insecure=True)

scan.scan_add(targets="x.x.x.x", name="Created via API") #, start="20170823T180000")
scan.scan_run()
{'source': 'GithubIssues', 'repo_name_with_owner': 'tenable/nessrest', 'issue_id': 248076053, 'repo_stars': 402.0}