Subject implementations

[benjchristensen]

http://msdn.microsoft.com/en-us/library/hh242969(v=vs.103).aspx

This issue is intended to cover the implementation of various Subjects as part of Rx.Net intended for placement in rx.subjects.

[benjchristensen]

The ReplaySubject was implemented in #218

[michaeldejong]

Hey I noticed this issue is still open. Did I forget to implement a Subject?

[benjchristensen]

Looks like they are now all done. Thank you @michaeldejong