Added the concept of plugin.
Added an empty plugin named CorePlugin that will host the core
functionality of Homewatcher.
Working on contexthandlers. Work in progress.
Enhanced error reported when pyknx v1 is installed.
When trying to run setup.py from Homewatcher while Pyknx version 1 is
installed (most likely by mistake, because the --pre option was not
passed to *pip install*), the error reported to the user was not clearly
pointing out the cause:
```
Running setup.py (path:/tmp/pip_build_root/homewatcher/setup.py)
egg_info for package homewatcher
    Traceback (most recent call last):
      File "<string>", line 17, in <module>
      File "/tmp/pip_build_root/homewatcher/setup.py", line 26, in
<module>
        import homewatcher
      File "homewatcher/__init__.py", line 23, in <module>
        from pyknx import Version
    ImportError: cannot import name Version
    Complete output from command python setup.py egg_info:
    Traceback (most recent call last):

  File "<string>", line 17, in <module>

  File "/tmp/pip_build_root/homewatcher/setup.py", line 26, in <module>

    import homewatcher

  File "homewatcher/__init__.py", line 23, in <module>

    from pyknx import Version

ImportError: cannot import name Version
```

The fix consist in importing ensurepyknx.py from homewatcher/__init__.py
to get a clear error.
Completed fix initiated in previous commit.
Was not completely fixed, ensurepyknx was requiring some additional
changes to properly check Pyknx's version.
Merge commit 'eff23a6'