Implemented the 'elementAt' and 'elementAtOrDefault' operators. see #41
Merge branch 'element_at' of git://github.com/zsxwing/RxJava into pull-390-element-at

Conflicts:
	rxjava-core/src/main/java/rx/Observable.java
Added missing license header
Merge pull request #399 from benjchristensen/pull-390-element-at

Merge Pull #390 - elementAt