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.
Read settings from environment variables

If a setting isn't configured in `Web.config` or `App.config`
(or `ServiceConfiguration.cscfg` for Azure applications), then
resort to reading it from the environment variables.
Merge branch 'fix/dotnet4-build'
Merge branch 'feature/environment-variables'