Add projectile-find-implementation-or-test-other-window

Enhances existing functionality by extracting
projectile-find-implementation-or-test from
projectile-toggle-between-implementation-and-test, so it can be used
both to toggle, or to open in other window.
Display error if buffer is not visiting a file

A sane error for when either
`projectile-toggle-between-implementation-and-test` or
`projectile-find-implementation-or-test-other-window` are used on a
buffer where the file name is nil.
Merge pull request #239 from dgtized/show-test-implementation-in-other-window

Add projectile-find-implementation-or-test-other-window