Fix #27

Fix for tricky case (#27)
```
2.1.1 :001 > Phonelib.parse '0000','PH'
NoMethodError: undefined method `[]' for nil:NilClass
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/phone_analyzer.rb:20:in `analyze'
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/phone.rb:24:in `initialize'
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/core.rb:108:in `new'
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/core.rb:108:in `parse'
    from (irb):1
    from /usr/local/rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'
2.1.1 :001 > Phonelib.parse '0000','IN'
NoMethodError: undefined method `[]' for nil:NilClass
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/phone_analyzer.rb:20:in `analyze'
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/phone.rb:24:in `initialize'
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/core.rb:108:in `new'
    from /usr/local/rvm/gems/ruby-2.1.1/gems/phonelib-0.3.0/lib/phonelib/core.rb:108:in `parse'
    from (irb):1
    from /usr/local/rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'
```