Add .gitignore file

Includes files generated by each of:
   * autogen.sh
   * configure
   * make

.gitignore

+# autogen.sh-generated files
+Makefile.in
+src/Makefile.in
+config.guess
+config.h.in
+config.sub
+configure
+depcomp
+install-sh
+ltmain.sh
+missing
+
+aclocal.m4
+m4/libtool.m4
+m4/ltoptions.m4
+m4/ltsugar.m4
+m4/ltversion.m4
+m4/lt~obsolete.m4
+
+# downloaded files
+gtest
+
+# in-tree configure-generated files
+Makefile
+src/Makefile
+config.h
+config.log
+config.status
+
+libtool
+protobuf-lite.pc
+protobuf.pc
+**/.deps
+stamp-h1
+
+# in-tree build products
+**/*.o
+**/*.lo
+**/*.la
+src/.libs
+
+**/.dirstamp
+
+**/unittest*.pb.*
+**/cpp_test*.pb.*
+
+src/protoc
+src/unittest_proto_middleman
+