Read settings from environment variables

If a setting isn't configured in `Web.config` or `App.config`
(or `ServiceConfiguration.cscfg` for Azure applications), then
resort to reading it from the environment variables.
Merge pull request #14 from jneill/feature/environment-variables

Read settings from environment variables when not configured elsewhere
Update certificate, add IsNullOrWhiteSpace

Update certificate, add IsNullOrWhiteSpace method for CORE35 so .NET 3.5
users won't fail on this block.
Remove  SSL Cert pinning

Remove SSL Cert pinning, bump version
Merge branch 'master' of github.com:logentries/le_dotnet
Merged changes from upstream and updated version.