Add `deps` command to list dependencies for a package without installing

I've created a `deps` command to resolve package dependencies and their versions.

It's useful for creating a requirements.txt file for later use with the --no-deps parameter, without having to install all the packages.

The usage and the solution are very similar to the `install` command using the --no-install parameter, collecting InstallRequirements objects and printing the package and version information.

It is working with a simple requirement, repository requirements and with requirements files.

Tests are included and the `deps-command` branch is based on the `development` branch.

Here is an usage sample and output to check flask dependencies:

```
$ pip deps flask
# downloading / unpacking / running messages ...
flask==0.10.1
Werkzeug==0.9.4
Jinja2==2.7.1
itsdangerous==0.23
markupsafe==0.18
```