[maven-release-plugin] prepare release gson-2.3.1

git-svn-id: http://google-gson.googlecode.com/svn/trunk@1310 2534bb62-2c4b-0410-85e8-b5006b95c4ae
[maven-release-plugin] prepare for next development iteration

git-svn-id: http://google-gson.googlecode.com/svn/trunk@1312 2534bb62-2c4b-0410-85e8-b5006b95c4ae
javadocs for 2.3.1

git-svn-id: http://google-gson.googlecode.com/svn/trunk@1313 2534bb62-2c4b-0410-85e8-b5006b95c4ae