Better build system

- configure script added
- demo/*.cc are now compiled (the executables are in demo/build)
- version number appears in the doc and automatically updates
- the cross reference to the lemon doc can be relocated (e.g. to a local
copy of the doc)
- some repo reorganization has taken place
- better .hgignore

 syntax: glob

 *.obj

 *.orig

 *.rej

 *~

 *.o

 *.log

 *.lo

 *.tar.*

 *.bak

 *.tag

 *.tmp

 .dirstamp

 Doxyfile

 Makefile

 syntax: regexp

 (.*/)?\#[^/]*\#$

 (.*/)?\.\#[^/]*$

 ^html/.*

 ^gen-images/.*

 ^gen-dox/.*

 ^demo/build/.*