change version name to comply with PEP440 and resolve recurrent pbs with pip

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branch : python3-transition
change version to comply with PEP440 and resolve pip pbs
mention one-plugin-per-module limit in troobleshooting for https://sourceforge.net/p/yapsy/bugs/14/
fix PluginInfo properties and add unit-tests for this now not-so trivial class
see also: https://sourceforge.net/p/yapsy/bugs/13/
a little more santy check to fix https://sourceforge.net/p/yapsy/bugs/12/
fix ConfigurablePluginManager.loadplugin bug reported at https://sourceforge.net/p/yapsy/bugs/17/
merge bug fixes from default branch

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branch : python3-transition
fix merge (py2>3)

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branch : python3-transition