Merge pull request #117 from chrqls/dev

[#111] Removing crash on MDPI format when we show the details of a question
Adding new styles for buttons
Merge commit '65b8544' into dev
[#111] Removing NullPointerException when displaying 'prepared questions' screen on mobile
[#111] Removing NullPointerException when displaying 'preview iqset' screen on mobile format
[#111] GeneralActivity startworking on mobile (new design)
[#111] Adding Spinner to filter correct answers in GeneralActivity
[#111] Removing NullPointerException when trying to display the details of a question on mobile format
Replacing ImageButton by Button when confirming the deletion of a question (for all formats)
Fixing layout bugs on mobile format + cleaning code (optimizing layouts, removing comments)
Optimizing 'detail of a student' layout on all format