IANAL
Use a union on ARM to ensure alignment for floating point loads

I hope this fixes ARM. I have no way to test.
fix ARM_ARCH stuff

I was so tired when I pushed that last patch I did not notice it was totally
half baked. This is a much better, and actualy tested version.
improve Sereal::Encoder's documentation

* add despription of header data structure
* encode_sereal() cannot encode header
* sereal_encode_with_object() can encode data + header
* added desciption of encode_sereal_with_header_data()
Perl: Add Ivan to author list