fasttime support

I have ported Simon's fasttime utility into lubridate. Lubridate parser is now smart enough to detect POSIX strings. No R level function for posix parsing as yet. I will have to think how to do it nicely.

It is still experimental and one (extremely unlikely) user test for heterogenuous parsing fails. So you will have to activate it by setting fasttime option. This is what I have now:

```
system.time(out <- as.POSIXct(tt, tz = "UTC"))
   user  system elapsed 
  9.732   0.156   9.893 
> system.time(out <- ymd_hms(tt))
   user  system elapsed 
  6.760   0.344   7.110 
> options(lubridate.fasttime = T)
> system.time(out <- ymd_hms(tt))
   user  system elapsed 
  1.688   0.460   2.152 
> 
```

Actual parsing takes only 0.17 seconds. The rest is POSiXlt <> POSIXct conversion. I guess it might be avoided but would require building core lubirdate code around POSIXct instead of POSIXlt. I will give it a try in a couple of months if I have time. 

Would you mind giving write access to the repo? Most of the parser/guesser/stamp code is mine. It would be very convenient if I could modify that code without going through pulls. Thanks.


fix #198