Update ChangeLog.md and installation_index.html

The installation document now points to a stable version i. e. the one
tagged as 0.9.1 in the git repository.
Change __status__ and __version__ strings

These strings were updated to reflect a Development status and the
current version 0.9.1-dev.
Merge pull request #201 from ElDeveloper/update_0.9.1-dev

Update 0.9.1 dev