Create segmental_tree.c

similar with interval tree implementation.
See reference:
http://www.cse.iitk.ac.in/users/aca/lop12/slides/06.pdf
http://wenku.baidu.com/view/8de8d953f01dc281e53af031.html
http://wenku.baidu.com/view/f9077440be1e650e52ea9903.html
http://www.cnblogs.com/shuaiwhu/archive/2012/04/22/2464583.html
segmental tree could be used to calculate the interval cover problem, searching in several intervals or special sum problem.