add SkString::append(const char c)

tests/StringTest.cpp

Index: tests/StringTest.cpp
===================================================================
--- tests/StringTest.cpp	(revision 9077)
+++ tests/StringTest.cpp	(working copy)
@@ -93,6 +93,18 @@
     REPORTER_ASSERT(reporter, a.equals("hello world") && a == e && a == f);
 
     a.reset();
+    a.append(SkString("string"));
+    a.append("text");
+    a.append('c');
+    REPORTER_ASSERT(reporter, a.equals("stringtextc"));
+
+    a.reset();
+    a += 'c';
+    a += "text";
+    a += SkString("string");
+    REPORTER_ASSERT(reporter, a.equals("ctextstring"));
+
+    a.reset();
     b.resize(0);
     REPORTER_ASSERT(reporter, a.isEmpty() && b.isEmpty() && a == b);