Add a dev module to run Gitiles from an embedded Jetty server

The various ways of running WARs from maven directly have proven to be
a headache and not work reliably on seemingly-identical environments.
Instead, reduce the amount of Maven magic to a minimum, and use
Jetty's nice embedding support to start a server serving only a
GitilesServlet from /*.

Change-Id: Ib07c5f94cb76e7cd0208096591e3e7cc6148f1ac

pom.xml

diff --git a/pom.xml b/pom.xml
index 4c0b85e..049c528 100644
--- a/pom.xml
+++ b/pom.xml
@@ -37,6 +37,7 @@
   </properties>
 
   <modules>
+    <module>gitiles-dev</module>
     <module>gitiles-servlet</module>
     <module>gitiles-war</module>
   </modules>