fix #99, can play in background, play any song at any way !

after doing lots of trial and errors, it seems that the code hanged on
node-webkit's request function, and based on nw's different context's
mechanism, I force the code to run under Node's context and this did
fix the problem
Merge pull request #118 from EragonJ/bug/99

Partial fix #99, can play in background, play any song at any way! (bug still rarely happens)