Add IconicFontButton and supporting code.

[niedzielski]

This PR adds support for a new class, IconicFontButton. It's an Android button with built in support for an IconicDroid background, customizable via plain XML. It's a little rough around the edges, but worked for my purposes. That said, I'm happy to make changes. If we merge this, we should consider updating the demo to show the new functionality simply by adding the following to a layout file:

<com.atermenji.android.iconicdroid.IconicFontButton
    style="@style/IconicFontButton"
    app:iconUtf="@string/awesome_plus_square_o"
    app:font="awesome" />

[niedzielski]

Rebased. I realize this commit is a starting point but I think it's a valuable direction for the library to go. Let me know what you think or changes are needed to get this in. Thanks.

[atermenji]

@niedzielski thanks a lot, I'll take a look at your commits soon, when I'll manage to find some free time