Put back autoloading of classes in ns + workaround for ROOT-6094

Merge of two commits:

Revert "Revert "Revert "Revert "Enable autoloading of classes in namespaces""""

This reverts commit 16d1b2c0db79ba64eed134b6f65b1a07734fb72c.

Workaround for ROOT-6094

Some facts:
1) Autoloading of classes in namespaces requires empty namespaces
in the rootmaps
2) If a namespace exists *empty* and a library is loaded whic has
overloaded functions in that namespace,  from within PyROOT, those
overloaded functions cannot be used.
3) We workaround this adding an empty anonymous namespace within
the namespaces to allow the usage of such functions.