Add ability to optionally replace AlertDialog.Builder with android.support.v7.app.AlertDialog.Builder

[kjeremy]

For material design goodness!

[alexshikov]

+1

[alexshikov]

How about use AppCompat by default? #20

[brianchance]

I have not done any android development in about a year, I also do not have a valid license to compile and push. I am willing to merge the PR, but am not sure I can push out a nuget package out without the compiled dll's.

[kjeremy]

Can you use the starter edition?

[alexshikov]

Would be great if we can find out the way of publishing new versions, because I'm about to create another Pull-Request with iOS crash fix.

[brianchance]

Right now I am also traveling. I will merge both pull requests, if you can get me binaries checked into the lib folder, I should be able to create and upload a nuget package.