Expand file names before processing them

We need to expand a file name to make it absolute and to process any special symbols before trying to process it.

Fixes an error from projectile-switch-project, when default-directory contains a tilde, e.g. "~/", as is the case in *scratch* and probably other special buffers:

```
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  directory-file-name(nil)
  projectile-parent("~")
  projectile-locate-ancestor-containing("~" ".svn")
  projectile-svn-project-root("~/")
  projectile-project-root()
  projectile-dirconfig-file()
  projectile-maybe-invalidate-cache(nil)
  projectile-find-file(nil)
  #<subr call-interactively>(projectile-find-file nil nil)
  ad-Advice-call-interactively(#<subr call-interactively> projectile-find-file nil nil)
  apply(ad-Advice-call-interactively #<subr call-interactively> (projectile-find-file nil nil))
  call-interactively(projectile-find-file nil nil)
  command-execute(projectile-find-file)
```