ListView entries in single layouts (Fixed #14)

The homework data is now stored in "ArrayList<HashMap<String, String>>", and not in a single String array like before. This have some
advantages:
- better design in ListView (now a LinearLayout, ported by F-Droid application, thank you therefore)
- ID for deletion is now directly gotten from HashMap and do not have to be stored in SharedPreferences
 -> Import/Export now only imports/exports Homework.db
- now it's easy to solve the following problems:
 -> #12 Edit Homework
 -> #13 Mark homework as completed
 -> #15 Sort Homework
"Providing Up Navigation", or home as up

as shown here: http://developer.android.com/training/implementing-navigation/ancestral.html
added ContextMenu and removed OnItemClickListener
fixed bug on tablets with home as up

Source: http://stackoverflow.com/questions/5015094/how-to-determine-device-screen-size-category-small-normal-large-xlarge-usin/19256468#19256468
moved some functions in own classes
changes in preferences order
Big Revise
Worked on code:

- TODO to README.md
- deleted empty files in res
- "startActivityForResult" to "startActivity"
- renamed some variables
- BuildInfo now in Utils.java
- AddHomework.java revised
- and more...
Now licensed under GPLv3
deleted SharedPreferences in Helper.Homework
added static function in Helper.Homework for deletion of a single
homework
now able to edit a homework (fixes #12)

sources:
http://stackoverflow.com/a/819427
http://stackoverflow.com/a/4063183
changed credit summary and deleted translations
added function to change language in app (fixes #26)

Sources:
// Override language in HWManager.java
http://roosmaa.net/application-wide-locale-override/
// Get device Locale
http://stackoverflow.com/a/4212417
// Get app Locale
http://sevennet.org/2014/12/04/how-to-get-the-current-language-in-device/
fixed bug in edit function

forgot to add the old until date to the until String
imported translations from Crowdin
added latest build