This idea has already been tried many times in the past. Rosin core solder has a problem of eating away the metal barrel required to extrude it, and layers will not lay down on top of themselves correctly. Pushing solder through a hot nozzle is not a new idea.
@ThantiK
recommended modification?
You can find out when you can link to his kickstarter. 
Build log available here. Chime in, i’m sure he would love some feedback.
Looks like someone’s already chimed in about the build log’s apparent misrepresentation of the work of others as it’s own. http://dustsreprap.blogspot.co.nz/2011/03/restrapping-cartesian-bot.html
Nah, he took some some pictures of someone else’s j-head and pretended they were of his solder extruder, or so the fellow whose link I posted claimed… :-/
He updated the pictures to more realistic ones. But i doubt he get a kickstarter funding with these results.
http://hackaday.io/project/998-Affordable-Metal-3d-Printer