Build and install libtac as shared library

Additional install public headers for libtac shared library.
SO-Versioning is initially set to 0:0:0.
debian: Introduce shahred library packaging libtac
Introduce pkg-config file for libtac
Fix include directory in pkg-config file

All headers are stored in the libtac sub-directory.
Merge pull request #13 from gollub/shared_libtac

[RFC] Build and install libtac as shared library