issue #46 : Build Error

[heshamsafi]

include/suffix_tree.h :
    - fixed build errors ( access modifiers issue)
    - removed typedef struct ... as it is unnecessary in C++
src/suffic_tree_demo.cpp :
    - fixed typo in function declaration name (seperate -> separate)

[heshamsafi]

I did not know whether to push this to master branch or develop branch.
so if you want it on the other branch ... let me know.