Include x86 and/or 64 bit version of MuPDF library

[intrications]

As mentioned in #264, we are not including the x86 version of MuPDF. Is this just for APK size reasons?

More tablets are now running on x86, although they (all) will convert the ARM code on the fly so the x86 library isn't essential.

[zhouhao27]

The x86 is much faster in emulator.

[intrications]

If this is done then it would be worth looking into create separate APKs for each architecture to keep the APK size as small as possible.

http://developer.android.com/google/play/publishing/multiple-apks.html

64 bit support (e.g. on the Nexus 9) is another reason for this issue:

http://android-developers.blogspot.co.uk/2014/10/getting-your-apps-ready-for-nexus-6-and.html

Can test in a 64 bit emulator:

https://plus.google.com/+AndroidDevelopers/posts/XG1WmNDMe8H

[intrications]

Create separate APKs with gradle:

http://ph0b.com/android-studio-gradle-and-ndk-integration/

[intrications]

http://stackoverflow.com/questions/29955330/mupdf-android-jni-library-is-very-large