Updates to README and code maintainability.

Lots of improvements in code maintainability and updates in README and the FilePicker API. Thanks to alirezaaa for all the hard work and all the pull requests!

Bug fixes

• Addresses issue where target sdk is lower then the build sdk (i.e. running the library on an Android L device that was build against api level 23), this resulted in no files being displayed.

Bug fixes and enhancements

• Fixed issues with android marshmallow (hopefully)
• Localized all strings into values->strings.xml

Fixed jitpack build error.

Fixed an error causing Jitpack builds to fail.

New Features and Bug Fixes

Hopefully fixed some bugs with the header in the list view. Also added a new material version of the file picker activity. Also added a builder to clean up the intent creation and removed file size calculation from directories. Will only happen on files or when you click the info button.

Official Release!

First release of FilePickerLibrary on Jitpack.io! I'm permanently switching all my libraries to this because it is much easier and more reliable.