Update LPlaceholderTextView.m

The purpose of these changes is to make LPlaceholderTextView allow the changing of the placeholder text/color.

1. Move setting of _placeholderLabel.textColor out of creation our of the creation of _placeholderLabel.

2. Add setters for placeholderText and placeholderColor that (in addition to setting the variable) call setNeedsDisplay to force a redraw.