Search bar filtering and sorting by file size in the file browser

as #12,

add UISearchDisplayController to FLEXFileBrowserTableViewController. and implement related methods for search.



The common pattern is something like
```objc
@implementation NSMutableArray (FLEXStack)

- (void)flex_push:(id)anObject
{
...
}

- (id)flex_pop
{
...
}

@end
```

https://developer.apple.com/library/ios/documentation/cocoa/conceptual/ProgrammingWithObjectiveC/CustomizingExistingClasses/CustomizingExistingClasses.html#//apple_ref/doc/uid/TP40011210-CH6-SW4