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
Sort methods on Array, List<T>, and other core collections accept null comparers. This is not the case for immutable collections, despite what the doc comments claim. This change fixes the inconsistency buy allowing null comparers being passed to Sort methods of the immutable collections.
Update ImmutableArray`1+Builder.cs
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
Add directory links for SR.cs files to they appear under the Resources folder in VS.
Merge pull request #143 from weshaggard/LinkSR

Add directory links for SR.cs files so they appear under the Resources folder in VS
Pointed to corefx contrib guide on wiki instead of microsoft/dotnet
Merge pull request #153 from davkean/master

Pointed to corefx contrib guide on wiki instead of microsoft/dotnet
Remove unnecessary allocations

While looking at string allocations in our code base,
I noticed a couple that can be trivially avoided in the XML libraries.
Merge pull request #151 from KrzysztofCwalina/dotnet2

Removes some string-related allocations from XML libraries
Merge pull request #140 from KrzysztofCwalina/dotnet1

Allow null comparer to be passed to ImmutableArray<T>.Sort