Ability to detect long press on hyperlink

SETextView上でのリンクの長押しが知りたかったので追加しました.
タッチ/クリックが始まると同時に NSTimer をスタートさせる微妙なやり方ですが, [ここ](https://github.com/questbeat/SECoreTextView/blob/85f0ff1f00b2d035bed3f6840f0a175eedc0aab8/Lib/SETextView.m#L697-735)の実装が間違っていなければ特に問題ないかと思います.
iOS/Mac OS両方で動作確認しました.

## New Methods in SETextViewDelegate
    - (BOOL)textView:(SETextView *)aTextView didLongPressOnLink:(SELinkText *)link atIndex:(NSUInteger)charIndex;
Tells the delegate that the link was long pressed in the specified text view.

## New Properties in SETextView
### Public
    @property (nonatomic, assign) NSTimeInterval minimumLongPressDuration;
The minimum period fingers/mouse must press on the link to be recognized as long press.

### Private
@property (nonatomic, weak) NSTimer *longPressTimer;
@property (nonatomic, assign, getter = isLongPressing) BOOL longPressing;