Change arg to be optional in projectile-find-dir

For consistency with projectile-find-file.  This is necessary so that projectile-find-dir can be used as value of projectile-switch-project-action.
Merge pull request #185 from deprecated/patch-1

Change arg to be optional in projectile-find-dir
Correct spelling of "implementation" (missing "t")

Rename function from `projectile-toggle-between-implemenation-and-test` to `projectile-toggle-between-implementation-and-test`
Add "C-c p t" to command list in README.md
Merge pull request #189 from deprecated/patch-1

Fix typo in function name and update README
Fix indentation
Correct grammar on not in project error message
Merge pull request #190 from lee-dohm/error-message

Correct grammar on not in project error message
[Fix #191] It seems you still have to specify action even if you have coerce.