Added the two variants of the first operator.
Added FirstOrDefault operation
Added link urls to the msdn descriptions
Merge branch 'first-firstDefault' of git://github.com/jmhofer/RxJava into operator-first-merge

Conflicts:
	rxjava-core/src/test/java/rx/ObservableTests.java

This merges pull request https://github.com/Netflix/RxJava/pull/357

Also aliased first with takeFirst to match takeLast.
Merge pull request #362 from benjchristensen/operator-first-merge

Merge 'first' Operator