adding marble diagrams to javadocs; standardizing formatting & terminology
Merge branch 'docs'
Merge remote-tracking branch 'origin/master'
Add contravariant for min and max
Merge pull request #496 from zsxwing/min-max-contravariant

Add contravariant for min and max
Implement the scheduler overloads for Range, From, StartWith
Merge pull request #492 from zsxwing/scheduler-overloads

Implement the scheduler overloads for Range, From, StartWith
Update Observable.java

from(scheduler) -> add diagram, standardize javadoc comments
range(scheduler) -> add diagram, standardize javadoc comments
startWith(scheduler) -> add diagram, standardize javadoc comments