fixing full profile field names

I had errors when using this library, they were occurring when it was getting the user's full profile:
- `Unknown field {last-modified-timstamp} in resource {Person}`
- `Unknown field {first-proposal-comments} in resource {Person}`
- `Unknown field {_url} in resource {MemberUrl}`

After checking the http://developer.linkedin.com/documents/profile-fields I've changed the names for profile fields according to it and it should work now.