[fix api] Expose `checkFile` and fix logical condition
[fix api] Expose `checkFile` and fix logical condition

When `checkFile` is set to `false`, `forever` doesn't check if script
file exists. This is useful for spawning node with parameters like
`--trace-gc`:

    var forever = require('forever');

    var m = new forever.Monitor(['node', '--trace-gc', 'gc.js'], {
      checkFile: false
    });
    m.start();

saisai/forever,jotham/forever,rbramwell/forever,ryan321/forever,JJediny/forever,jotham/forever,rakyll/forever,arnabsinha4u/demo-documentation,kethle/forever,JJediny/forever,kethle/forever,Jonekee/forever,rbramwell/forever,arnabsinha4u/demo-documentation,ContinuumIO/forever,jqk6/forever,nguyen-tien-mulodo/forever,foreverjs/forever,saisai/forever,vcarrera/forever,ryan321/forever,yyx990803/forever,foreverjs/forever,ContinuumIO/forever,nguyen-tien-mulodo/forever,rakyll/forever,jqk6/forever,yyx990803/forever,Jonekee/forever,vcarrera/forever