Made a Julian date conversion



Hey there karbarcca, I wanted a Julian Date conversion, e.g.

```
# Julian conversion
Base.Test.@test datetime(2452695.625) == datetime(2003, 2, 25, 3)
Base.Test.@test datetime(2456630.375) == datetime(2013, 12, 3, 21)
```

so I made one. Sorry for the redundancy with the issue, this is my first pull request! The code is not as golfy as it could be; it was mostly ganked from some javascript...