Fix issue with getting ghar_dir when sys.argv[0] is a relative path.

Fix issue when ghar invoked with relative path ('bin/ghar' or 'ghar/bin/ghar')
or when 'ghar' invoked in ghar/ dir and relative path 'bin' is in the PATH
(e.g. when using Boxen).

Credit goes to github users @Flaeme and @danny316p for the fix.

## Issue Description

I was playing with the github Boxen project.  Unfortunately, it adds a relative path for 'bin' at the start of my PATH.  This exposes a problem with ghar when sys.argv[0] returns a relative path like 'bin/ghar' instead of the full absolute path the code expects. This results in ghar incorrectly reporting 'No repos found' or saying 'We can't yet handle non-ghar symlinks'.

sys.argv[0] may return a relative path when invoking 'ghar' within 'ghar/' dir and the relative path 'bin' is in PATH or when user invokes ghar with a relative path (e.g 'bin/ghar' or 'ghar/bin/ghar').

## Issue Test Cases
    mouse:~/ghar (fix-relative-bin-path-issue) $ git checkout philips-ghar/master
    Note: checking out 'philips-ghar/master'.
    ...

### Relative path issue
    mouse:~/ghar ((13db390...)) $ bin/ghar status
    No repos found in
    Please read /README
    mouse:~/ghar ((13db390...)) $ bin/ghar install --status
    No repos found in
    Please read /README
    mouse:~/ghar ((13db390...)) $ cd ..
    mouse:~ $ ghar/bin/ghar status
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    dotfiles-git: dirty
    dotfiles-hg: clean
    dotfiles-shell: dirty
    mouse:~/ghar ((13db390...)) $ cd ghar
### 'bin' in PATH issue
    mouse:~/ghar ((13db390...)) $ PATH=bin:$PATH
    mouse:~/ghar ((13db390...)) $ ghar status
    No repos found in
    Please read /README
    mouse:~/ghar ((13db390...)) $ ghar install --status
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    We can't yet handle non-ghar symlinks.
    dotfiles-git: has no dotfiles
    dotfiles-hg: has no dotfiles
    dotfiles-shell: has no dotfiles

## Issue Test Cases After Fix
    mouse:~/ghar ((13db390...)) $ git checkout fix-relative-bin-path-issue
    Previous HEAD position was 13db390... Merge pull request #10 from githubaff0/fix-link-subcommand-ignores-repo-args
    Switched to branch 'fix-relative-bin-path-issue'
### Relative path issue
    mouse:~/ghar (fix-relative-bin-path-issue) $ bin/ghar status
    dotfiles-git: dirty
    dotfiles-hg: clean
    dotfiles-shell: dirty
    mouse:~/ghar (fix-relative-bin-path-issue) $ bin/ghar install --status
    dotfiles-git
     ok	/Users/githubaff0/.gitconfig
    dotfiles-hg
     ok	/Users/githubaff0/.hgignore_global
    dotfiles-shell
     ok	/Users/githubaff0/.bash_profile
     ok	/Users/githubaff0/.bashrc
    mouse:~/ghar (fix-relative-bin-path-issue) $ cd ..
    mouse:~ $ ghar/bin/ghar status
    dotfiles-git: dirty
    dotfiles-hg: clean
    dotfiles-shell: dirty
    mouse:~/ghar (fix-relative-bin-path-issue) $ cd ghar
### 'bin' in PATH issue
    mouse:~/ghar (fix-relative-bin-path-issue) $ ghar status
    dotfiles-git: dirty
    dotfiles-hg: clean
    dotfiles-shell: dirty
    mouse:~/ghar (fix-relative-bin-path-issue) $ ghar install --status
    dotfiles-git
     ok	/Users/githubaff0/.gitconfig
    dotfiles-hg
     ok	/Users/githubaff0/.hgignore_global
    dotfiles-shell
     ok	/Users/githubaff0/.bash_profile
     ok	/Users/githubaff0/.bashrc