Switch from RCurl to curl

[leeper]

This reduces dependencies (namely methods and bitops) and will hopefully mean a performance enhancement.

• DESCRIPTION
• NAMESPACE imports
• Code updates
  • Switch RCurl::curlEscape and RCurl::curlUnescape to curl::curl_escape and curl::curl_unescape
  • Find replacement for RCurl::base64Encode. Maybebase64::base64encode`

[jeroen]

Simon's base64enc package is by far the best base64 implementation available. There is an implementation available in the openssl package as well.

[leeper]

Thanks, @jeroenooms!