adding test for issue 72

Added a short test for issue 72.  It is generating errors of the form:

```
  1) Failure:
TestTimecop#test_datetime_nanoseconds_are_preserved [timecop_test.rb:466]:
<#<DateTime: 2013-07-15T17:33:49-07:00 ((2456490j,2029s,528206000n),-25200s,2299161j)>> expected but was
<#<DateTime: 2013-07-15T17:33:49-07:00 ((2456490j,2029s,528206110n),-25200s,2299161j)>>.
```
Notice that the nanoseconds are getting truncated (528206000n vs 528206110n).

P.S.: Yep, this is my first pull request.  Hope I got it right.