[Added] libs folder to gitignore, in order not to push my compiled
version of CLIPS
[Modified] Moved clips file to assets folder to pack it with the app
[Modified] Necessary code in MainActivity to make it work with the new
clips file location

Reason of this change:
- Moving the files to an external memory in devices where it is emulated
is inconvenient
- App should not depend on what the user does and doesn't do with his/her
external memory

How the new method works:
1. File is packed with the app
2. App unpacks the file by reading it from assets folder and creates an
accesible copy in the app's private file folder
3. The path to that file is passed to the clips environment