Add user option to composer::project

[mbrodala]

ATM composer::project is evaluated with user root which has disadvantages:

1. Usually SSH keys for repository access are not set up for root but for a regular user instead: create-project command fails on private repositories.
2. The created directory is owned by root: ownership has to be changed to an unprivileged user.

Thus a $user option (which may default to $composer::params::user) should be added which is applied to all exec resources within composer::project.

[igalic]

+1
@mbrodala do you wanna propose your own pr?

[mbrodala]

Sure but first I'd like to get #9 merged to avoid rebasing issues. ;-)

[igalic]

let's get this one in too, and then make a new release!

[mbrodala]

I'll give it a try in the next days. :-)