chore(Makefile): add Makefile

Add simple 'Makefile' to compile '.lean' files into '.olean'
files. It assumes that 'lean' is in the 'PATH' variable. If this is
not the case, you can run 'make' while specifying the pathname for
'lean' as follows:

    $ make LEAN=~/work/lean0.1/build/shell/lean
chore(.travis.yml): compile .lean to .olean and upload to dropbox