Add an .editorconfig file

Just a start. Could help enforcing the Coding Style
https://github.com/Microsoft/dotnet/blob/master/CONTRIBUTING.md#c-coding-style

What is .editorconfig:
http://editorconfig.org/#overview

Plugins for IDEs to support:
http://editorconfig.org/#download
Added trim_trailing_whitespace = true
Apply settings only to *.cs files

Refs https://github.com/dotnet/corefx/pull/56#issuecomment-63719328
Added indendaton rules for *.csproj and *.nuspec

Refs https://github.com/dotnet/corefx/pull/56#issuecomment-63198357
Add Rebuild target to build.proj
Update all the XML projects to aways set DebugSymbols=true to ensure pdbs also get generated for Release builds
Remove comment
Re-add two spaces as default indent

Refs https://github.com/dotnet/corefx/pull/56#issuecomment-64101144
Merge pull request #145 from weshaggard/AddRebuild

Add Rebuild target to build.proj
Merge pull request #146 from weshaggard/DebugSymbolsForRelease

Update all the XML projects to aways set DebugSymbols=true to ensure pdb...
Merge pull request #56 from ravage84/editorconfig

Add an .editorconfig file