Added support for static content like HTML pages or XML

[Ellerbach]

Added support for String type of content like HTML or script or XML. Created a similar type or call back like functions but adding content_type. Modify the send_http_headers() function to support the new type. Created a new command = 'p' Made support for content_type = "application/json" for all exisiting code Had to move the addStringToBuffer function up in the file before the send_http_headers function as it was needed to encode the content_type.

Added an example which deliver a simple HTML page which derives from the base ESP8266 one. Example shows how to embedded an encoded base64 image and output simple page including it plus the exising variable and the command parameter.