Added a check option to allow the testing of a Puppetfile

```bash
=>> r10k puppetfile check
{alevy@toodles-galore r10k} [11:45:44] |ruby-1.9.3@puppet-3.x| (lindenle_check %)
 =>> echo $?
0
{alevy@toodles-galore r10k} [11:45:47] |ruby-1.9.3@puppet-3.x| (lindenle_check %)
 =>> emacs Puppetfile
{alevy@toodles-galore r10k} [11:45:51] |ruby-1.9.3@puppet-3.x| (lindenle_check %)
 =>> r10k puppetfile check
ERROR: Puppetfile bad syntax
/Users/alevy/vcsworking/r10k/lib/r10k/puppetfile.rb:52:in `instance_eval'
/Users/alevy/vcsworking/r10k/lib/r10k/puppetfile.rb:52:in `load!'
/Users/alevy/vcsworking/r10k/lib/r10k/puppetfile.rb:44:in `load'
/Users/alevy/vcsworking/r10k/lib/r10k/cli/puppetfile.rb:65:in `block (2 levels) in command'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/gems/cri-2.3.0/lib/cri/command.rb:296:in `call'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/gems/cri-2.3.0/lib/cri/command.rb:296:in `run_this'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/gems/cri-2.3.0/lib/cri/command.rb:249:in `run'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/gems/cri-2.3.0/lib/cri/command.rb:262:in `run'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/gems/cri-2.3.0/lib/cri/command.rb:262:in `run'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/gems/r10k-1.0.0/bin/r10k:7:in `<top (required)>'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/bin/r10k:19:in `load'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/bin/r10k:19:in `<main>'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/bin/ruby_noexec_wrapper:14:in `eval'
/Users/alevy/.rvm/gems/ruby-1.9.3-p448@puppet-3.x/bin/ruby_noexec_wrapper:14:in `<main>'
```