Add 'execSync' option

[ttback]

If execSync=true, run the 'rename' task synchronously

This is a necessary flag in order to run 'watch' task after this rename task. Tested locally against latest grunt-contrib-watch v0.6.1.

An issue is also opened on the grunt-contrib-watch repo, gruntjs/grunt-contrib-watch#409

Usage example:

rename: {
           options:{
                execSync: true
           },
           foo: {
               src: 'foo/src'
               dest: 'foo/dest'
           }
}

[jdavis]

This plugin is on the way to being deprecated; I don't use Node/Grunt much anymore.