fix: #81 TaskView._detect_url run into infinite loop under GTK 3.14.6

This bug report describes why this patch is needed for users who are using latest GTK+ release 3.14.x https://bugzilla.gnome.org/show_bug.cgi?id=742000

The original way to test whether to reach the end of text buffer by comparing the iterator's offset is not stable in GTK+ 3.14.x.

Without this patch, some tasks cannot be opened and GTG would crash due to the infinite loop.