Fast implementation of Hamming weight

The target method: System.Reflection.Internal.BitArithmetic.CountBits
Old implementation uses 24 arithmetic operations
New fast implementation uses 12 arithmetic operations
Replace '? :' operator with '??'
Merge pull request #160 from LukaszWolanin/master

Replace '? :' operator with '??'
Adding more direct link to the .NET Foundation Forums
Merge pull request #167 from Petermarcu/forum_link

Adding more direct link to the .NET Foundation Forums
Reduce Node sizes by shrinking "height" from Int32 to Byte
Merge pull request #168 from stephentoub/immutable_node_size

Reduce Node sizes by shrinking "height" from Int32 to Byte
Limit application of the JitIntrinsicAttribute

This attribute should limit where it can be placed to something that the
JIT will respect.
Merge pull request #169 from briangru/master

Limit application of the JitIntrinsicAttribute
Adding badges for pull request and issue times.
Merge pull request #173 from Petermarcu/adding_badges

Adding badges for pull request and issue times.
Merge pull request #155 from AndreyAkinshin/FastHammingWeight

Fast implementation of Hamming weight
Add System.Linq.Parallel Product and Test Code
Merge branch 'master' of https://github.com/dotnet/corefx
Update README.md to include PLinq description.

Reviewed by @joshfree and @krwq.
Update casing of Linq -> LINQ