repository: Add a gitignore file
repository: Add default CMAKE_BUILD_TYPE

For each packages, we assign by default the Debug mode for
CMAKE_BUILD_TYPE if the CMAKE_BUILD_TYPE is empty.

We add -ggdb3 if CXX is GNU GCC and CMAKE_BUILD_TYPE equals Debug or
RelWithDebInfo and CFLAGS options -Wall -Wextra if CXX is GNU GCC.
celldevs: fix memory leaks

The CellDevs extension have several memory leaks in set functions. We
fix them (closes #13).
decision: add missing standard include

The decision/Plan.cpp needs sstream instead of istream.
repository: upgrade packages with API changes

The vle::devs::ExternalEventList is now a typedef to std::vector <
vle::devs::ExternalEvent* > instead of a class. The function addEvent is
missing. We replace with a call to push_back.