Relicense JUnit from CPL to EPL

EPL is the successor version of the CPL this makes re-licensing
possible without seeking the approval of all contributors.

CPL is actively hurting JUnit adaptation, it the following cases the
deciding factor against JUnit was CPL. In both cases EPL would have
been fine.
* Netbeans can't ship JUnit [4]
* OpenJDK uses TestNG instead of JUnit [5]

Mike Milinkovich from the Eclipse Foundation wrote [1]

> Back in 2009, the CPL was superseded by the EPL. This means that the
> EPL is the successor version of the CPL. It also means that using the
> CPL is the licensing equivalent of using deprecated code.
>
> Because the EPL is the successor version to the CPL, the "new version
> re-licensing" clause in Section 7 of the CPL applies. In other words,
> you can re-license your project without seeking the approval of all
> of your contributors.
> The CPL and EPL basically differ by about one sentence, which you can
> see here. The difference relates to the scope of patent licenses
> terminated should someone sue another party for patent infringement.
> This is the kind of stuff that lawyers love, but most developers
> don't really care about.

On the migration from CPL to EPL Mike Milinkovich wrote [2]

> There was a two step process that was followed to make this happen.
> First, following the terms of the CPL, IBM assigned the
> responsibility to serve as the Agreement Steward of the CPL to the
> Eclipse Foundation. Second, the Eclipse Foundation officially
> recognized the EPL 1.0 as the new version of the CPL 1.0. In OSI
> license terminology, the EPL now supersedes the CPL.
>
> A quick read of the two licenses will quickly show that they are very
> very close. Other than their names and (previously) their Agreement
> Stewards, the only substantive difference is the breadth of the
> patent license termination in the event of a patent law suit. (See
> the second paragraph of Section 7.) For more information on the
> relationship between the CPL and the EPL see the EPL FAQ.

You can find more information about the difference at [3].

The new `LICENSE.txt` is based on a plain text version of the EPL with
formatted so that the diff is a clean as possible.

I'll submit another pull request for the gh-pages branch.

 [1] http://mmilinkov.wordpress.com/2013/02/13/jruby-moves-to-the-epl/
 [2] http://mmilinkov.wordpress.com/2009/04/16/one-small-step-towards-reducing-license-proliferation/
 [3] http://www.eclipse.org/legal/eplfaq.php#EPLDIFFER
 [4] https://netbeans.org/community/releases/70/relnotes.html#deprecated
 [5] http://openjdk.java.net/jtreg/build.html