KT-3715 make Collection.notEmpty a property

Renamed "notEmpty()" methods to "isNotEmpty()" in following classes:
    - Collection
    - String
    - Array

Added "notEmpty" property to Collection class
KT-3715 updated java-doc

"not" was missing in the java-doc for "notEmpty" property
Merge pull request #296 from mhshams/master

KT-3715 make Collection.notEmpty a property