Florian Wiesweg authored Apr 13, 2017 and Guillaume Roguez committed Apr 19, 2017

Whenever possible, functionality of underlying standard
libraries should be preferred to custom implementations
to prevent maintenance cost and security issues.

- Remove custom base64 implementation
- Replace by pjlib's [1]

[1] http://www.pjsip.org/pjlib-util/docs/html/group__PJLIB__UTIL__BASE64.htm

Change-Id: I0d111c69e93f352a7b4e599bf97fecb968447c28