Merge pull request #159 from mairbek/lastOrDefault

Implemented LastOrDefault Operator
Merge remote-tracking branch 'mairbek/single'
Added @throws javadocs
Suppressed rawtypes warnings

Conflicts:
	language-adaptors/rxjava-groovy/src/test/groovy/rx/lang/groovy/ObservableTests.groovy
	rxjava-core/src/main/java/rx/Observable.java
Update README.md
Merge pull request #160 from benjchristensen/mairbek_single-merge

Manual merge of mairbek/single Pull #157
Merge pull request #161 from garethbowles/master

Added build status badges