Remove duplicate program in examples/

I'm sure restrangetest is a fine program but we don't need to attempt
to build it twice.
git-ignore some more build products
Remove droplet-3.0.pc

This file is generated from droplet-3.0.pc.in at configure time, we
don't need to have a copy checked into version control.
Add Doxygen support to build system

Uses existing JavaDoc-style comments in the code to generate API
documentation using the widely available Doxygen tool.  Two sets of
documentation are produced, HTML under doc/html/ and UNIX man pages
under doc/man/.  A new docs: Makefile target will rebuild all the
Doxygen documentation from scratch.  No documents are installed or
distributed yet.

Added the @file directive to all the source which had existing JavaDoc
comments, as Doxygen wants this.
Update JavaDoc comments in conn.c
Update JavaDoc comments for dict.c
Make the dpl_dict_t docs a Doxygen module

which is slightly easier to read and doesn't expose the filename
"dict.c" in the documentation.
Document dpl_init() dpl_ctx_new() et al.