open both plain text and gzipped gnucash files

spec/gnucash_spec.rb

@@ -1,3 +1,12 @@
 describe Gnucash do
-
+  describe '.open' do
+    it 'opens a gzipped gnucash book' do
+      book = Gnucash.open("spec/books/sample.gnucash")
+      book.is_a?(Gnucash::Book).should be_true
+    end
+    it 'opens a plain text gnucash book' do
+      book = Gnucash.open("spec/books/sample-text.gnucash")
+      book.is_a?(Gnucash::Book).should be_true
+    end
+  end
 end