Fix the error message when incorrect number of arguments passed.

Before this change, if there were too little arguments e.g.
`trousseau set abc`, it would print a "Not enough argument supplied ..."
message which is correct but it would print the same message if there
were too many arguments e.g. `trousseau get abc 123`. Also, it should
have been arguments, not argument. This change makes the error message
more accurate.
Fix debian repo info in README

The original example will fail because of "Permission denied" (the scope of sudo does not apply
to the stdout append). This variant will work for non-root users.

Also modified it to the best practice of adding custom repositories to their own files within
/etc/apt/sources.list.d/foobar.list
Merge pull request #21 from colszowka/patch-1

Fix debian repo info in README
Add travis file
Merge pull request #19 from mtahmed/master

Fix the error message when incorrect number of arguments passed.
Merge branch 'master' into develop
Add password flag registration as global option [ref #13]
Update password flag to fetch it's default value from env [ref #13]
Add constants for trousseau password env var name [ref #13]
Update decryption mecanism to use password from global option [ref #13]

This is basically a workaround the global password option system.
A huge refactoring should be made later on to get rid of the whole
environment struct thing.
Update README instructions for password option [fix #13]
Merge branch 'feature/use_password_option' into develop