Fix pac-help output. Remove first ocurrence of '-'.

Before:
-------

```
$ pac
usage:
  pac <command> [...]

commands:
  ...
  pac ls outdated
```

Now:
----

```
$ pac
usage:
  pac <command> [...]

commands:
  ...
  pac ls-outdated
```
Merge pull request #2 from frodsan/fix_pac_help

Fix pac-help output. Remove first ocurrence of '-'.
Use `command -v` instead of `which`.

Based on https://github.com/pote/gpm/pull/24.
Merge pull request #3 from frodsan/use_command

Use `command -v` instead of `which`.
Use builtin whatis -p instead of command -v
Don't assume lua libraries
Avoid extra calls to mkdir -p
Remove ./lib as the default install location
Detect conflicting versions
Describe general purpose usage
Use builtin whatis -p instead of command -v.
Merge pull request #4 from frodsan/patch-1

Use builtin whatis -p instead of command -v.