Internalize and Document USD/BTC exchange rate

* tickerJob records the Bitstamp vwap every 5 minutes by default.
* Noted caveats of USD/BTC exchange rate handling.
Print lastBlockJob and watchedPaymentsJob intervals during startup
More README.md improvements, mainly security
Reorganizing Baron project files and filenames
  * Renamed the following files:
    * bitcoinutil.js -> lib/bitcoinrpc.js
    * invoicehelper.js -> lib/invoices.js
    * paymentutil.js -> lib/payments.js
    * invoicewebhooks.js -> lib/webhooks.js
    * db.js -> db/index.js
  * Deleted unused static resources
  * Added lib folder
Removed expiration from barontester test invoices
  * Removed expiration timestamp so we dont run into problems with the
    tester if the test invoices expire in the future.
Add TRUST_PROXY option to obtain client address from X-Forwarded-For