Added test for nested tuples.
Merged include/system/meta.h with include/mpl.h and moved to include/utility/meta.h;
Removed unused metaprograms from system/meta.h;
Metafunction EPOS::IF renamed to EPOS::conditional;
Moved every class from namespace std to namespace EPOS;
Moved tuple_test.cc and meta_test.cc to src/utility.