With this change, we bundle Reactive Extensions from Microsoft.

Steps to do:

- Until we add submodule, check out Rx sources from http://rx.codeplex.com:

	$ cd external
	$ git clone git://github.com/atsushieno/rx.git
	$ cd rx
	$ git checkout rx-oss-v1.0
	$ cd ../..

  Note that the original repo at rx.codeplex.com will *fail* on Linux!
  codeplex.codeplex.com/workitem/26133
  Also note that rx.codeplex.com is huge and takes very long time to checkout.

- expand rx-mono-changes-3.tar.bz2

	$ tar jxvf rx-mono-changes-3.tar.bz2

- Apply changes to mcs/class/Makefile:

	$ cd mcs/class
	$ patch -i add-rx-libs.patch -p3
	$ cd ../..

Then it should be done.

Note that this does not include Mono.Reactive.Testing into the build yet -
this library depends on nunit.framework.dll but it wouldn't be built before
this assembly is built. This needs to be resolved.

** Current Status

- We don't have Microsoft.Reactive.Testing.dll. Instead, I created an
  alternative Mono.Reactive.Testing.dll which *mostly* uses MS sources for
  that assembly but uses NUnit.Framework instead.

  To make it happen, I added a small script that automatically replaces
  MSTest dependency parts with that for NUnit (replacer.sh under rx tree).

  (We'll also have to rename namespaces and have more source changes, but
  so far it is to get things runnable.)

- To check the build sanity, I imported unit tests (as explained above)
  and it is supposed to run by "make run-test" in Mono.Reactive.Testing
  directory (the tests were all in one place in MS tests, so I made it
  in Mono.Reactive.Testing directory instead).