Fix plugin install code

Fix following errors:

1. On plugin install:
Traceback (most recent call last):
File ".\picard\ui\options\plugins.py", line 133, in open_plugins
self.install_plugin(path)
File ".\picard\ui\options\plugins.py", line 138, in install_plugin
dest = os.path.join(self.tagger.user_plugin_dir, file)
AttributeError: 'Tagger' object has no attribute 'user_plugin_dir'

2. On Open Plugin Folder:
Traceback (most recent call last):
File ".\picard\ui\options\plugins.py", line 151, in open_plugin_dir
QtGui.QDesktopServices.openUrl(QtCore.QUrl(self.loader %
self.tagger.user_plugin_dir, QtCore.QUrl.TolerantMode))
AttributeError: 'Tagger' object has no attribute 'user_plugin_dir'.

Code missed from commit 7c4fdf765cd404ba0742f2513d039a5b69e8a71d on
2013-06-08.
Fix error when installing over existing plugins

As reported in PICARD-444, the following error is received when
installing over an existing plugin:

Traceback (most recent call last):
File ".\picard\ui\options\plugins.py", line 134, in open_plugins
self.install_plugin(path)
File ".\picard\ui\options\plugins.py", line 148, in install_plugin
self.tagger.pluginmanager.install_plugin(path, dest)
File ".\picard\plugin.py", line 198, in install_plugin
same_file = os.path.samefile(path, dest) if dest_exists else False
AttributeError: 'module' object has no attribute 'samefile'
Add explanatory comment
Add News.
Merge branch 'sophist_fix_plugin_install' of git://github.com/Sophist-UK/picard into Sophist-UK-sophist_fix_plugin_install

Conflicts:
	NEWS.txt