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
I am fine to add the pattern, but I think it is enough to add the pattern _once_ at the root level. It will ignore any files in the sub-directories (just as the ```.DS_Store``` matches any such file in any subdirectory)