BUG: Constrain date parsing from strings a little bit more #4601

Currently dateutil will parse almost any string into a datetime.  This
change adds a filter in front of dateutil that will prevent it from
parsing certain strings that don't look like datetimes:

1) Strings that parse to float values that are less than 1000
2) Certain special one character strings (this was already in there,
   this just moves that code)

Additionally, this filters out datetimes that are out of range for the
datetime64[ns] type.  Currently any out-of-range datetimes will just
overflow and be mapped to some random time within the bounds of
datetime64[ns].
Merge branch 'fix-float-into-datetime' of https://github.com/danbirken/pandas into danbirken-fix-float-into-datetime