Install documentation to $(docdir)/julia instead of $(datarootdir)/julia/doc

[nalimilan]

This is the standard directory, distributions prefer documentation to go to $(docdir)/julia [1], which defaults to /usr/share/doc/julia. I'll have a look, this is mostly a reminder for myself.

By the way, by default calling make && make install you get the .rst installed directly. Maybe it would make more sense to compile this to HTML? Installing Python and configuration files does not make much sense either.

The man page is also duplicated in $(datarootdir)/julia/doc/man. Not sure why.

1: http://www.gnu.org/software/make/manual/html_node/Directory-Variables.html