Fixed images of student list on rollcall module
Updated build tools and Google Play Services
Replaced Gradle version from '+' to specific version
Updated build tools to version 21.1.1
Added call to ws function getAttendanceEvents
Merge branch 'develop' into feature/rollcall

Conflicts:
	app/src/main/java/es/ugr/swad/swadroid/modules/rollcall/RollcallConfigDownload.java
	build.gradle
Now the QR code is generated from the user nickname
Completed EventsDownload module
Completed EventsDownload module
Added UsersDownload module
Added UsersSend module
Added UsersSend module
Optimized imports
Fixed Logcat trace when user birthday is null
Completed GenerateQR module with "only nickname" modification
Updated back button code
Completed Rollcall module code
Set unmarked users as absent
Changed events icon
Added refresh of users list on return from scan mode