Add install and uninstall targets.

First you may have to do:

$ sudo chown -R $(whoami) /usr/local/bin/

Then just do:

$ ninja install
$ which btcd
/usr/local/bin/btcd

Or (after installing):

$ ninja uninstall
$ which btcd
empty # it should not return the above path.

Signed-off-by: Thiago Farina <tfarina@chromium.org>