Fix font size when FontAwesomeText uses style

Hi,

I was trying to style the ```FontAwesomeText``` with ```style``` attribute.
```xml
    <com.beardedhen.androidbootstrap.FontAwesomeText
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:duplicateParentState="true"
        android:id="@+id/icon"
        style="@style/MenuItemIcon"/>
```
This results in ```NullPointerException``` in the ```FontAwesomeText```.initialise on
```java
Matcher m = PATTERN_FONT_SIZE.matcher(xmlProvidedSize);
```

I found a solution. Instead of "regexping" the font size, obtain it as a dimension from the ```TypedArray``` object as following

```java
float scaledDensity = getContext().getResources().getDisplayMetrics().scaledDensity;
fontSize = a.getDimension(R.styleable.FontAwesomeText_android_textSize, 14.0f*scaledDensity)/scaledDensity;
```

It works with the ```textSize``` defined implicitly in ```style``` and with also with the explicitly defined ```textSize``` after the fix.

Peter