ENH: get client ID and client secret from environment variables

[kaczmarj]

It would be good to include logic in eapp/app.js to grab clientID and clientSecret from the environment if it exists. If those environment variables do not exist, get this information from the config file. For GitHub clientID and clientSecret, we can use GITHUB_CLIENT_ID and GITHUB_CLIENT_SECRET. This format can be extended for other services like GMAIL_CLIENT_ID and DROPBOX_CLIENT_ID.

[smpadhy]

The logic should grab ClientID and SecretKey environment variable set in Travis and pass it to app as environment variable and write it to the config file while building distributable app.