Add links to MIT's Algorithms Design courses. (#2806)

* Add links to two editions of MIT's Design and Analysis of Algorithms
course in MIT's Open Courseware from Spring 2012 and Spring 2015.
* Modify "Introduction to Algorithms" from Fall 2005 entry to follow
the same naming scheme as the two newly added courses.

free-courses-en.md

@@ -52,7 +52,9 @@
 
 * [Berkeley University CS 61B: Data Structures](http://datastructur.es/sp16/)
 * [IIT Bombay Foundation of Data Structures (CS213.1x)](https://courses.edx.org/courses/course-v1:IITBombayX+CS213.1x+1T2017/course/)
-* [Introduction to Algorithms (SMA 5503)](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-046j-introduction-to-algorithms-sma-5503-fall-2005) - Charles Leiserson
+* [MIT's Design and Analysis of Algorithms (Spring 2012)](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-046j-design-and-analysis-of-algorithms-spring-2012) - Dana Moshkovitz, Bruce Tidor
+* [MIT's Design and Analysis of Algorithms (Spring 2015)](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-046j-design-and-analysis-of-algorithms-spring-2015) - Erik Demaine, Srini Devadas, Nancy Lynch
+* [MIT's Introduction to Algorithms (SMA 5503) (Fall 2005)](https://ocw.mit.edu/courses/electrical-engineering-and-computer-science/6-046j-introduction-to-algorithms-sma-5503-fall-2005) - Charles Leiserson, Erik Demaine
 * [Princeton University Algorithms, Part 1](https://www.coursera.org/learn/algorithms-part1)
 * [Princeton University Algorithms, Part 2](https://www.coursera.org/learn/algorithms-part2)
 * [Stanford University Algorithms: Design and Analysis, Part 1](http://online.stanford.edu/course/algorithms-design-and-analysis-part-1)