Fix indexing logic

An incorrect folder variable was used for the purpose of indexing a
project via external commands.

This fixes #116. It also fixes #117.
Refactor a bit projectile-project-files

The name of the function was confusing since it was not actually
computing the project files for a while now, so it got renamed to
`projectile-dir-files`. A couple of helper functions were also extracted
from it.
Smarter check for project VC

If the project root contains a VC directory it's considered the project's VC.
Strip the properties of the default grep argument (fixes #119)
Strip the properties of the default replace argument
Don't show hidden buffers in projectile-project-buffers

Hidden buffers have names starting with ' '(a space) and are of no
interest to most users, so it makes sense for Projectile to filter them
out.

This fixes #118.
Introduce helper function projectile-thing-at-point

It wraps `(thing-at-point 'symbol)` and handles properly nil values.

This fixes #120.