Add extra View constructor so it can be included directly in XML

Solves this issue:
https://github.com/Polidea/android-zoom-view/issues/2

Example usage:
        <pl.polidea.view.ZoomView
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            />

Example crash without this fix:

Caused by: android.view.InflateException: Binary XML file line #15: Error inflating class pl.polidea.view.ZoomView
            at android.view.LayoutInflater.createView(LayoutInflater.java:603)
            at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:696)

<snip>

     Caused by: java.lang.NoSuchMethodException: <init> [class android.content.Context, interface android.util.AttributeSet]
            at java.lang.Class.getConstructorOrMethod(Class.java:472)
            at java.lang.Class.getConstructor(Class.java:446)