Use helm-projectile in switch to project action

[obar]

Use helm-projectile interface immediately after switching to a project with helm-projectile-switch-project.

This is a very small change, in addition to the file listing shown before, it gives the user their project buffers and recentf as sources after switching projects. It's the same interface but a little more convenient.

Closes #142

---

Before submitting a PR make sure the following things have been done (and denote this by checking the relevant checkboxes):

:question: The commits are consistent with our contribution guidelines

• [X] The new code is not generating bytecode or M-x checkdoc warnings
• [x] You've updated the changelog (if adding/changing user-visible functionality)
• [x] You've updated the readme (if adding/changing user-visible functionality)

[obar]

Thanks for your help! Hope this is better.