Fixed help information

[JohanMabille]

Fixes #8

[ianthomas23]

Looks good to me:

$ ./build/git2cpp -h
Git using C++ wrapper of libgit2 


./build/git2cpp [OPTIONS] [SUBCOMMAND]


OPTIONS:
  -h,     --help              Print this help message and exit 
  -v,     --version           Show version 

SUBCOMMANDS:
  init                        Explanation of init here 
  status                      Show modified files in working directory, staged for your next 
                              commit 
  add                         Add file contents to the index 
  branch                      List, create or delete branches 
  checkout                    Switch branches or restore working tree files 
$ ./build/git2cpp status -h
Show modified files in working directory, staged for your next commit 


./build/git2cpp status [OPTIONS]


OPTIONS:
  -h,     --help              Print this help message and exit 
  -s,     --short             Give the output in the short-format. 
          --long              Give the output in the long-format. This is the default. 
  -b,     --branch            Show the branch and tracking info even in short-format.