Support .NET Platform Standard

[andriysavin]

Need to add support for .NET Platform Standard for easier referencing from netstandard libraries.
Here's is motivation for this: I tried to reference the Rx PCL package from a netstandard library with "imports" trick usage, but I wasn't able to build that library - many errors arise when adding "imports", like LINQ becomes "invisible" to the compiler (all that happens when targeting netstandard1.4 and lower). And after all, this trick is more for third parties which don't hurry to adopt recent .Net trends; Rx, on the other hand, is Microsoft's child and should not hesitate to do that.

This seems to be relatively easy to do (at least for the first step), since there is a PCL version (Profile259) of Rx which is compatible with netstandard1.0. If I understand this document correctly, those PCL binaries should just be copied to a netstandard1.0 directory when producing packages, and also dependencies should be specified.

[clairernovotny]

Dup of #148

[SuperJMN]

Support for .NET Standard profiles. Including 1.5!

[clairernovotny]

completed!