Restrict bevy dependency to bevy_ecs etc

[mrchantey]

Restriction of bevy dependency to sub crates as discussed in #18

• bevy_ecs, bevy_app etc have been added as dependencies
• bevy references have been replaced with the corresponding sub crate
• bevy has been moved from dependencies to dev-dependencies as its being used by docs
• references in docs have been left alone, maybe we want to keep using bevy there for readability?