Make it possible for confidential clients to refresh tokens.  Also, update the web sample app to use the current middleware that has been separated from the core express package.
Update doc comment on acquireTokenWithClientCredentials to indicate that the new clientSecret parameter is optional.
Merge pull request #25 from RandalliLama/dev

Make it possible for confidential clients to refresh tokens.
Add standard ADAL ID request headers.
Merge pull request #26 from RandalliLama/dev

Add standard ADAL ID request headers.
Update changelog and version in prep for next release.
Merge pull request #27 from RandalliLama/dev

Update changelog and version in prep for next release.
Merge pull request #28 from AzureAD/dev

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