Prevent ninja from running install, uninstall and clean when no targets are specified.

As it was written, if you typed only 'ninja', it would run all the above
commands. Which is not good.

We can override this behavior by using a default target statemt.
We also used a special rule, called 'phony' to create an alias for
bin/btcd, so the user does not have to waste typing 'bin/' everything,
which is tedious.

This information can be found in ninja's manual:
martine.github.io/ninja/manual.html#_more_details

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