added cache validation support for datasource
added cache clearing
some issue fixed
fix npe
gradle updated
some issue fixed
Merge branch 'releases/1.3.4' of https://github.com/IstiN/android_xcore into xcore_2.0

Conflicts:
	xcore-library/xcore/build.gradle
	xcore-library/xcore/src/main/java/by/istin/android/xcore/service/AbstractExecutorService.java
	xcore-library/xcore/src/main/java/by/istin/android/xcore/service/DataSourceService.java
some issues fixed