add func InsertFilter(pattern string, pos int, filter FilterFunc) *App to replace AddFilter
pos can be const:
    BeforeRouter = iota
    AfterStatic
    BeforeExec
    AfterExec
    FinishRouter
replace filterPos to pos
eliminated improper comments
Merge pull request #314 from shavac/master

add function InsertFilter to replace AddFilter by using int const to determine filter position