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.
Show full path within project for files.
Add option for including top-level dir
Merge pull request #193 from m00natic/show-path

Show full path within project for files.
Document options for project switching behavior
Whitespace fixes
Improve python project type detection.
Merge pull request #195 from proofit404/master

Improve python project type detection.
Merge pull request #194 from deprecated/top-level

Support for using projectile-find-dir as switching action
Test project respect compilation-read-command value.
Add projectile-vc-dir bound to C-c p v

Define new function projectile-vc-dir which runs vc-dir on the project's root directory.  Bind this to C-c p v by default.
Merge pull request #197 from proofit404/master

Test project respect compilation-read-command value.