[docs] added some doxygen groups

I think the title says it all.

Doxyfile

@@ -106,9 +106,7 @@ HHC_LOCATION           =
 GENERATE_CHI           = NO
 BINARY_TOC             = NO
 TOC_EXPAND             = NO
-DISABLE_INDEX          = NO
 ENUM_VALUES_PER_LINE   = 4
-GENERATE_TREEVIEW      = NO
 TREEVIEW_WIDTH         = 250
 GENERATE_LATEX         = NO
 LATEX_OUTPUT           = latex
@@ -168,7 +166,7 @@ TEMPLATE_RELATIONS     = NO
 INCLUDE_GRAPH          = YES
 INCLUDED_BY_GRAPH      = YES
 CALL_GRAPH             = YES 
-CALLER_GRAPH           = NO
+CALLER_GRAPH           = YES 
 GRAPHICAL_HIERARCHY    = YES
 DIRECTORY_GRAPH        = YES
 DOT_IMAGE_FORMAT       = png
@@ -181,3 +179,7 @@ DOT_MULTI_TARGETS      = NO
 GENERATE_LEGEND        = YES
 DOT_CLEANUP            = YES
 SEARCHENGINE           = NO
+
+DISABLE_INDEX = YES
+GENERATE_TREEVIEW = YES
+

evhtp.h

@@ -285,6 +285,7 @@ struct evhtp_alias_s {
 };
 
 /**
+ * @ingroup evhtp_core
  * @brief main structure containing all configuration information
  */
 struct evhtp_s {
@@ -552,6 +553,11 @@ struct evhtp_ssl_cfg_s {
 };
 #endif
 
+/**
+ * @defgroup evhtp_core Core evhtp functions 
+ * @{
+ */
+
 /**
  * @brief creates a new evhtp_t instance
  *
@@ -561,8 +567,18 @@ struct evhtp_ssl_cfg_s {
  * @return a new evhtp_t structure or NULL on error
  */
 EVHTP_EXPORT evhtp_t * evhtp_new(evbase_t * evbase, void * arg);
+
+
+/**
+ * @brief free a evhtp_t context
+ *
+ * @param evhtp
+ *
+ * @return 
+ */
 EVHTP_EXPORT void      evhtp_free(evhtp_t * evhtp);
 
+/** @} */
 
 /**
  * @brief set a read/write timeout on all things evhtp_t. When the timeout
@@ -576,6 +592,7 @@ EVHTP_EXPORT void      evhtp_free(evhtp_t * evhtp);
 EVHTP_EXPORT void evhtp_set_timeouts(evhtp_t * htp, const struct timeval * r, const struct timeval * w);
 
 
+
 /**
  * @brief during the request processing cycle, these flags will be used to
  *        for query argument parsing. i.e., what to parse and not to parse.