Added box-sizing to the CodeMirror class

The editor view in Side By Side does not wrap lines properly (ie. they sometimes overflow beneath the live preview). Adding box-sizing property seems to fix this issue.

src/css/simplemde.css

@@ -1,4 +1,5 @@
 .CodeMirror {
+    box-sizing: border-box;
     height: auto;
     min-height: 300px;
     border: 1px solid #ddd;
@@ -325,4 +326,4 @@
 
 .CodeMirror .CodeMirror-placeholder {
     opacity: .5;
-}
+}