Check command

Some fixes for your `check` command.

* Normalize names to lowercase
* Tell `get_installed_distributions` not to skip any packages like `setuptools`, etc.

##### Before

```
$ pip check
pdbpp 0.7.2 requires pygments, which is not installed.
bpython 0.13.1 requires pygments, which is not installed.
devpi-web 2.2.2 requires pygments, which is not installed.
distribute 0.7.3 requires setuptools, which is not installed.
Jinja2 2.7.3 requires markupsafe, which is not installed.
pyramid 1.5.2 requires setuptools, which is not installed.
pyramid 1.5.2 requires zope.deprecation, which is not installed.
waitress 0.8.9 requires setuptools, which is not installed.
zope.interface 4.1.1 requires setuptools, which is not installed.
```

##### After

```
$ pip check
pyramid 1.5.2 requires zope.deprecation, which is not installed.
```

The latter is correct in my situation because my virtualenv had `Pygments` and `setuptools` installed but not `zope.deprecation`.