Add information to Time.parse ArgumentError

The reference manual of Ruby 1.9.2 does not clearly say that date="" raise an ArgumentError.
According to https://github.com/ruby/ruby/tree/v1_9_2_381, Time.parse raises ArgumentError when no data information.
The reference manual of 1.9.2 and later versions will show ArgumentError with the argument date="".
Merge pull request #52 from monkey-mas/add-information-to-Time.parse_argumentError

Add information to Time.parse ArgumentError