Pyreadr

* librdata source updated to 615dee09955b318ce128e0bfcc9d50aa3c9b7b7c
fixes 10 and reading files produced on windows with non ascii characters in content.
* Manifest file changed to include windows dll and those are taken from setup.py
for unix, this fixes installation error messages on macos when using brew.

* librdata source updated to b7ca1252b670d60a6e8d3c8ce6fcc8ab3b43d6ab
fixes 3.

* setup.py fixed to build on conda-forge.
* conda forge package released

* fixed distinguishing between empty string and NaN/NaT/None when writing.
* librdata src updated to latest sources as 2019.01.19

* fixed distinguishing between empty string and NA when reading.

* writing support added.