Allow dtype option to from_records constructor
It would be nice to be able to specify `dtype` of the various columns to a `DataFrame` in the `DataFrame.from_records()` constructor. Mostly because this is the constructor used by Spark in its [toPandas()](https://github.com/apache/spark/blob/master/python/pyspark/sql/dataframe.py#L1366) method.

Something like this would make it easier to either preserve type information from a Spark dataframe, or take advantage of pandas types without having to load dates, categoricals, etc as objects first.
