Update README.md

Update the download page link from the deleted wiki page to release page.
Update Windows Azure to Microsoft Azure
Microsoft Azure Pack is actually still called Windows Azure Pack, even though Windows Azure is now Microsoft Azure
Update windowsazure.com to azure.microsoft.com
Merge pull request #2460 from andrerod/master

Microsoft Azure Pack is still called Windows Azure Pack
Merge branch 'release-0.8.1' of github.com:WindowsAzure/azure-sdk-tools into release-0.8.1
update changelog for 0.8.1
update the date
Merge pull request #2498 from guangyang/release-0.8.1

update changelog and readme