Add code coverage using lcov and gcov

To generate the code coverage report you need gcov and lcov installed
and then execute

 ./configure --enable-code-coverage
 make check-code-coverage

 Writing directory view page.
 Overall coverage rate:
   lines......: 76.5% (10519 of 13759 lines)
   functions..: 85.5% (491 of 574 functions)
   branches...: 57.5% (7040 of 12250 branches)
 file:///home/user/asn1c/asn1c-0.9.28-coverage/index.html

Open the index.html with a web browser
diff --git a/.gitignore b/.gitignore
index da42ba2..2c69308 100644
--- a/.gitignore
+++ b/.gitignore
@@ -68,3 +68,6 @@
 # /skeletons/tests/
 /skeletons/tests/check-*
 
+#code coverage
+*.gcno
+*.gcda