add validation before running, and inform users that it's started

before
```
main
~ xclisten
/Users/orta/.rvm/gems/ruby-2.0.0-p247/gems/xclisten-0.0.3/lib/xclisten.rb:30:in `project_name': undefined method `split' for nil:NilClass (NoMethodError)
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/gems/xclisten-0.0.3/lib/xclisten.rb:13:in `initialize'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/gems/xclisten-0.0.3/bin/xclisten:32:in `new'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/gems/xclisten-0.0.3/bin/xclisten:32:in `block in <top (required)>'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/gems/xclisten-0.0.3/bin/xclisten:32:in `fork'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/gems/xclisten-0.0.3/bin/xclisten:32:in `<top (required)>'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/bin/xclisten:23:in `load'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/bin/xclisten:23:in `<main>'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/bin/ruby_noexec_wrapper:14:in `eval'
	from /Users/orta/.rvm/gems/ruby-2.0.0-p247/bin/ruby_noexec_wrapper:14:in `<main>'
```

after 
```
~/s/r/xclisten (master ⚡=) xclisten
Please run xclisten from a directory with an xcworkspace.
```
and 

```
~/s/i/energy (toolbar_polish ↩) xclisten
Started xclistening to Artsy Folio.xcworkspace
```