Makefile.am: Use a single Makefile.am and parallel tests

Allow parallel building and testing by using a single Makefile.am

Implement parallel testing using TAP, with various drivers and
compilers living in the build/ directory.

Fix all sorts of issues that this caused, including builddir != srcdir,
leaks in tests and so on.

It would have been nice to break out all the above into separate
commits ... blush.
configure.ac: Bump version number
libsecret: Add secret_service_encode_dbus_secret() and opposite

Add secret_service_encode_dbus_secret() and
secret_service_decode_dbus_secret() functions for encoding
and decoding the Secret Service API DBus structs that carry
secrets on the wire.

These are not added to the stable or scripting APIs.
libsecret: Fix documentation typos