The following two commands, in CraftBukkit git-Bukkit-e8efbb8 via command blocks, will provide the player with an unbreakable diamond shovel that can break only grass blocks:

/minecraft:give @p minecraft:diamond_shovel 1 0 {CanDestroy:["minecraft:grass"],Unbreakable:1}
/summon Item ~ ~ ~ {Item:{id:minecraft:diamond_shovel,Count:1,tag:{CanDestroy:["minecraft:grass"],Unbreakable:1}}}

These work fine in the 1.8 build of craftbukkit, along with vanilla 1.8 (when you switch minecraft:give with give), but the 1.8 build of spigot only honors the Unbreakable NBT tag. Additionally, any item saved with a CanDestroy NBT tag on vanilla 1.8 or cb 1.8, and then loaded on spigot 1.8, will lose that tag. For testing purposes, this was done with an empty dir clean install, with default configs, and no plugins.