Target netstandard2.0

[zyzhu]

Various changes based on the initial efforts of @kblohm. Both Deedle and Deedle.RProvider compiles targeting dotnet standard now.

[zyzhu]

Trigger travis build.

[dsyme]

THis is great work. @kblohm Could you do a post-hoc review? thanks

[kblohm]

Sure i can.
Just a few minor notes:

• all the Tests should probably target netcoreapp2.1 instead of 2.0.
• the fix for the documentation-test does not work for non-english languages because the error messages get translated. I, for example, can not run them.
• i can not run the RPlugin-Tests or even compile them (not even on net45). Not sure if i am missing something there or if you have the same problem.
• in my opinion, the the RPlugin should not target netstandard until the RProvider actually supports it. At least if you plan on releasing a version 2.0.0-alpha soon, i would just release Deedle without it and provide the RPlugin later.

[zyzhu]

@kblohm Thanks for your review. I've done a pull request to address some problems. Please take a look and let me know what you think.
#393

1. • All Deedle tests target netcoreapp2.1 and net45
   • RPlugin.tests target net45
   • Documentation.tests target net461 because of fsharp.formatting
2. I don't know how to force locale of error messages to en-us. I've tried some tricks but didn't succeed. Maybe a short-term solution is to turn off documentation test on your local machine so that build can pass.
3. • RPlugin.Tests is really a pain. My environment has R-3.4.3 (with zoo package installed) installed at C:\R\R-3.4.3\bin\x64. I add this to path environment variable. I also set R_HOME to be C:\R\R-3.4.3. With those environment settings, RPlugin.Tests shall be able to build.
   • But in order to run RPlugin.Tests, I couldn't get it working via dotnet test as it always look for x86 version of R by default. For now, I enforced RPlugin.Tests to be run via nunit.consolerunner.
   • I also changed the default setting in build.fsx. By default it will only build allcore which exclude rplugin and rplugin tests so that we can focus on Deedle itself until the picture of RProvider is clearer.
4. I did remove netstandard from Deedle.RPlugin. We shall keep it at net45 until RProvider is on netstandard.