remove fs-cp dependency

[tunnckoCore]

resolves #19

[coveralls]

Coverage remained the same at 91.111% when pulling b515504 on tunnckoCore:master into 364bd29 on koajs:master.

[haoxins]

@tunnckoCore I only use node@4+, can you split support node@0.12 to another PR?

@jonathanong what's your option for 0.12?

[tunnckoCore]

I'm using this package in upcoming v2 (in a bit, I'm waiting this pr) of koa-better-body. And we need/want have support for 0.12. It's not serious to drop support for 0.12 because of the few variables and one arrow function. I hate such forces and non-logical moves - i'm not talking for this exactly case, in general. They doesn't do so special things and not worth using them here currently.

I was not going to add this commit, but it breaks my builds, so it's needed to me. If not accept this PR, close and I'll still use the fixed fork.

Cheers,
Charlike!

[haoxins]

OK, but we can use const on 0.12, right? Why changed to var?

[tunnckoCore]

Actually, yes we can. Maybe I didn't realize that I'm changing them or my habits, nevermind. What's the point using const exactly, currently here? There's no point.

I'm against using features and things (in general) because they are just there and because they exist, sorry.

[haoxins]

What's the point using const exactly

1. no need revert const to var again. It's noisy
2. they are constant indeed

[jonathanong]

won't be merging this PR in this form. please feel free to open a new one!