parser: added macros to check if it is a SIP or HTTP reply
gzcompress: enable compression for http messages

- tested replying with compressed body to an HTTP request from FireFox
core: aliased http_reply_hack to http_reply_parse

- a more relevant name for the core parameter that enables parsing http
  replies
gzcompress: default encoding header value set to deflate

- it is what UA use for this zlib compression algorithm
gzcompress: readme updated to reflect default encoding value to deflate
gzcompress: added a small usage example in docs
gzcompress: removed empty section in functions chapter

- use deflate overall config example