Fix memory leak for font assets

Each call to Typeface.createFromAsset will load a new instance of the typeface into memory,
and this memory is not consistently get garbage collected
    http://code.google.com/p/android/issues/detail?id=9904
(It states released but even on Lollipop you can see the typefaces accumulate even after
multiple GC passes)

You can detect this by running:
adb shell dumpsys meminfo com.your.packagenage

You will see output like:

 Asset Allocations
    zip:/data/app/com.your.packagenage-1.apk:/assets/Roboto-Medium.ttf: 125K
    zip:/data/app/com.your.packagenage-1.apk:/assets/Roboto-Medium.ttf: 125K
    zip:/data/app/com.your.packagenage-1.apk:/assets/Roboto-Medium.ttf: 125K
    zip:/data/app/com.your.packagenage-1.apk:/assets/Roboto-Regular.ttf: 123K
    zip:/data/app/com.your.packagenage-1.apk:/assets/Roboto-Medium.ttf: 125K

The workaround is to keep a reference to the loaded typeface and reuse it
Merge pull request #101 from teslacoil/typefacehelper

Fix memory leak for font assets