Git ignore IntelliJ configuration files
I often use IntelliJ for running static code analysis and creating diagrams to aid my understanding of Portfolio Performance. Perhaps other people do as well. This pull request just ignores some standard IntelliJ configuration files by adding them to `.gitignore` without changing any other files.

This includes the following:
* `.idea`
* `*.iws`
* `*.iml`
* `*.ipr`
.gitignore
target
workspace
.metadata/*
.project
.recommenders/*
.DS_Store
target
workspace
.metadata/*
.project
.recommenders/*
.DS_Store

### IntelliJ IDEA ###
.idea
*.iws
*.iml
*.ipr
Thank you. That is simple and elegant. I have committed and pushed the code.