mediaPlaybackRequiresUserAction unable to be set to "NO"

reported at: https://github.com/phonegap/phonegap-iphone/issues/318
by: https://github.com/splendid

It looks like the code that deals with checking the plist and setting the `mediaPlaybackRequiresUserAction` value is incorrect.

it makes the assumption that this value is set to "NO" by default (which is incorrect); and as such does not allow a user to set this value to "NO".

Relevant code: PhoneGapDelegate.m, lines 425-427:

```objective-c
if ([mediaPlaybackRequiresUserAction boolValue] && [self.webView respondsToSelector:@selector(mediaPlaybackRequiresUserAction)]) {
self.webView.mediaPlaybackRequiresUserAction = YES;
}
```

(Apologies for not submitting a patch myself, I'm still battling #104)