Add support for gems and improve RPM support

[pombredanne]

This PR adds support for Rubygems version comparison and improves RPM handling

Fixes for RPM:

• Fix the rpm tests #2
• https://github.com/sassoftware/python-rpm-vercmp/issues/2
• https://github.com/sassoftware/python-rpm-vercmp/issues/4

Fixes for Rubygems:

• rubygems is not semver #5

[pombredanne]

A typo to fix

[Gitii]

Hi @pombredanne,

I just came across your PR and wanted to add something briefly. I hope it helps you further.
In one of my side projects, I needed to port the vercmp stuff to c# and I also needed to adapt this part:
https://github.com/nexB/univers/blob/8b84788c7f43271bd10274785f315c05c476b61d/src/univers/rpm.py#L214

Instead of comparing the length of the ascii encoded numbers, I had to parse it and compare the actual numbers:

var m1_num = Int32.Parse(m1_head);
var m2_num = Int32.Parse(m2_head);

var cmp = m1_num.CompareTo(m2_num);

if (cmp < 0)
{
    return -1;
}

if (cmp > 0)
{
    return 1;
}

Together with your fixes (thanks!) all except one broken tests are green now (my tests are based on https://raw.githubusercontent.com/rpm-software-management/rpm/master/tests/rpmvercmp.at).
For example "1.0" < "2.0" was failing.

Edit:
All green now with this additional check after line https://github.com/nexB/univers/blob/8b84788c7f43271bd10274785f315c05c476b61d/src/univers/rpm.py#L221

else
{
    var head_cmp = String.Compare(m1_head, m2_head, StringComparison.Ordinal);

    if (head_cmp < 0)
    {
        return -1;
    }

    if (head_cmp > 0)
    {
        return 1;
    }
}

Maybe python works differently, but it will still help you.