Add .gitignore file with Visual Studio defaults

Taken from https://github.com/github/gitignore/blob/master/VisualStudio.gitignore

(specifically from 20cf6d5dcd5411f950d78d0007ae2be53474978b)
Remove binary files
Fix build when NET4_0 symbol is defined

The build was broken when the NET4_0 symbol was defined: there was
a typo in a variable name and a couple of using statements were
missing.
Merge pull request #13 from jneill/refactor/ignore-files

Remove and ignore build artifacts
Merge pull request #15 from jneill/fix/dotnet4-build

Fix build when NET4_0 symbol is defined