Allow creation of projects that use idiomatic golang directory structures

Currently it is difficult to create a project that uses the idiomatic golang directory structure like this:

    /bin
    /pkg
    /src
        /github.com
            /jerejones
                /hello
                    hello.go

This is because PsiDirectory.createSubdirectory can not create multiple levels at once. This pull request breaks up the project name if it contains path seperators and creates the directories one at a time.

This allows creating a project like this:
![go_new_project](https://cloud.githubusercontent.com/assets/1229385/4110542/e1857c98-31f3-11e4-806b-ce7cec491ee9.png)

To result in this:
![go_project_tree](https://cloud.githubusercontent.com/assets/1229385/4110544/e87b563a-31f3-11e4-866c-6b689ba149a2.png)