Split providers by smaller groups

There are 3 problems with the current separation (the one I did in #29):
 1. The providers get update too frequently.
 2. Old is too big, if your connection is not fast enough, composer will timeout and fail horribly.
 3. There is actually a bug, last-year is not really last year.

Problem 2 is the reason why I’m doing this. It is especially painful if wpackagist’s server gets overloaded.

Before:
```
+---------------+----------+---------+
| provider      | packages | size    |
+---------------+----------+---------+
| old           | 41388    | 4.5 MiB |
| last-year     | 19803    | 2.2 MiB |
| last-2-months | 9840     | 1.1 MiB |
| last-week     | 1054     | 117 KiB |
+---------------+----------+---------+
```

After:
```
+-----------+----------+---------+
| provider  | packages | size    |
+-----------+----------+---------+
| 2013      | 11216    | 1.2 MiB |
| 2012      | 9145     | 1.0 MiB |
| old       | 15046    | 1.6 MiB |
| 2011      | 7317     | 820 KiB |
| 2014-06   | 5355     | 603 KiB |
| 2014-09   | 9241     | 1.0 MiB |
| 2014-03   | 3861     | 433 KiB |
| 2014-12   | 8211     | 927 KiB |
| this-week | 2693     | 303 KiB |
+-----------+----------+---------+
```