[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();

lib/forever/monitor.js

@@ -51,6 +51,7 @@ var Monitor = exports.Monitor = function (script, options) {
   this.max         = options.max;
   this.killTTL     = options.killTTL;
   this.childExists = false;
+  this.checkFile   = options.checkFile !== false;
   this.times       = 0;
 
   //
@@ -212,7 +213,7 @@ Monitor.prototype.start = function (restart) {
 // trying to execute a script with an env: e.g. node myfile.js
 //
 Monitor.prototype.trySpawn = function () {
-  if (this.command === 'node' || (this.checkFile && !this.childExists)) {
+  if (this.command === 'node' && this.checkFile && !this.childExists) {
     try {
       var stats = fs.statSync(this.args[0]);
       this.childExists = true;
@@ -405,4 +406,4 @@ Monitor.prototype._getEnv = function () {
   });
 
   return merged;
-};
+};