Add the Str namespace for convenience function on strings
replace some Cmd_Tokenize with Cmd::Args
Minor changes to the previous commit

* Add Cmd::Args.size()
* Replace usage of Argc and Argv with [] and size when it is not used
for a command: we might want to introduce a BasicToken and make
Cmd::Args derive from it.
Cmd: add doc
Merge branch 'engine-upgrade' into nacl

Conflicts:
	CMakeLists.txt