Date generator

It is not clear to me how to shrink dates: bringing them towards 1970 (or the lower limit) does not seem to simplify much.
Another option would be to shrink in detail:
```
"2014-12-22T14:10:34"
"2014-12-22T14:10:00"
"2014-12-22T14:00:00"
"2014-12-22T00:00:00"
"2014-12-01T00:00:00"
"2014-01-01T00:00:00"
"1970-01-01T00:00:00"
```
:+1: always for the robustness principle but in some cases I fear that being too large (and the constructor of DateTime is very large) can be a mess.

User have to define a domain of Dates. A date is "Y-m-d H:i:s" which is uniquely determined while "10 seconds ago" is not uniquely determined, are we sure that this is a feature in the definition of the domain of Dates?
```php
$generator = date('10 seconds ago', '2500 days ago');
object(Eris\Generator\Date)#18 (3) {
  ["lowerLimit":"Eris\Generator\Date":private]=>
  object(DateTime)#16 (3) {
    ["date"]=>
    string(19) "2014-12-23 00:07:54"
    ["timezone_type"]=>
    int(3)
    ["timezone"]=>
    string(11) "Europe/Rome"
  }
  ["upperLimit":"Eris\Generator\Date":private]=>
  object(DateTime)#20 (3) {
    ["date"]=>
    string(19) "2008-02-18 00:08:04"
    ["timezone_type"]=>
    int(3)
    ["timezone"]=>
    string(11) "Europe/Rome"
  }
  ["intervalInSeconds"]=>
  int(-215999990)
}
```
I am not against the current implementation but I want that we think about this carefully. :-)