Implemented single and singleOrDefault methods
Merge branch 'master' into single
Updated single/singleOrDefault according to review request
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
Merge pull request #160 from benjchristensen/mairbek_single-merge

Manual merge of mairbek/single Pull #157