Move js files into src/

Test plan:
- Make sure huxley tests, jasmine tests, make build, make metrics, make test all
  still work.

Auditors: alpert

src/ParseError.js

@@ -0,0 +1,38 @@
+/**
+ * This is the ParseError class, which is the main error thrown by KaTeX
+ * functions when something has gone wrong. This is used to distinguish internal
+ * errors from errors in the expression that the user provided.
+ */
+function ParseError(message, lexer, position) {
+    var error = "KaTeX parse error: " + message;
+
+    if (lexer !== undefined && position !== undefined) {
+        // If we have the input and a position, make the error a bit fancier
+
+        // Prepend some information
+        error += " at position " + position + ": ";
+
+        // Get the input
+        var input = lexer._input;
+        // Insert a combining underscore at the correct position
+        input = input.slice(0, position) + "\u0332" +
+            input.slice(position);
+
+        // Extract some context from the input and add it to the error
+        var begin = Math.max(0, position - 15);
+        var end = position + 15;
+        error += input.slice(begin, end);
+    }
+
+    // Some hackery to make ParseError a prototype of Error
+    // See http://stackoverflow.com/a/8460753
+    var self = new Error(error);
+    self.name = "ParseError";
+    self.__proto__ = ParseError.prototype;
+    return self;
+}
+
+// More hackery
+ParseError.prototype.__proto__ = Error.prototype;
+
+module.exports = ParseError;