Abstracted XML parser library and switched to autotools building.

expat is still the default parser, but libxml2 can also be used
via an option to the configure script.  New parsers can easily be
added by implementing a parser_foo.c that uses the interface defined
in parser.h.

src/parser.h

@@ -0,0 +1,43 @@
+/* parser.h
+** strophe XMPP client library -- parser structures and functions
+**
+** Copyright (C) 2005-2009 Stanziq, Inc. All rights reserved.
+**
+**  This software is provided AS-IS with no warranty, either express or
+**  implied.
+**
+**  This software is distributed under license and may not be copied,
+**  modified or distributed except as expressly authorized under the
+**  terms of the license contained in the file LICENSE.txt in this
+**  distribution.
+*/
+
+/** @file
+ *  Internally used functions and structures.
+ */
+
+#ifndef __LIBSTROPHE_PARSER_H__
+#define __LIBSTROPHE_PARSER_H__
+
+#include "strophe.h"
+
+typedef struct _parser_t parser_t;
+
+typedef void (*parser_start_callback)(char *name,
+                                      char **attrs,
+                                      void * const userdata);
+typedef void (*parser_end_callback)(char *name, void * const userdata);
+typedef void (*parser_stanza_callback)(xmpp_stanza_t *stanza,
+                                       void * const userdata);
+
+
+parser_t *parser_new(xmpp_ctx_t *ctx, 
+                     parser_start_callback startcb,
+                     parser_end_callback endcb,
+                     parser_stanza_callback stanzacb,
+                     void *userdata);
+void parser_free(parser_t * const parser);
+int parser_reset(parser_t *parser);
+int parser_feed(parser_t *parser, char *chunk, int len);
+
+#endif /* __LIBSTROPHE_PARSER_H__ */