doc/conf.py: improve `release' string value

This _PLSDK_VERSION_ tag can be replaced with the actual
PLSDK version when generating the documentation included
in release archives.  Also the `version' value is now
re-used in `release' to avoid updating the package version
in two places.
fix plsdk.__version__ and use it everywhere

Fix __version__ value in plsdk/__init__.py and use it in setup.pu
and doc/conf.py to have the version number in a single place.
ChangeLog v1.5 -> v1.5.1