fix imports for twitter example

[shamaton]

This issue made for PR(#810)
This content is same.

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This commit has errors in twitter sample.
63262b2

So this PR fixed theirs.

errors

$ go run server.go
server.go:6:2: cannot find package "github.com/labstack/echo/recipe/twitter/handler" in any of:
    /usr/local/go/src/github.com/labstack/echo/recipe/twitter/handler (from $GOROOT)
    ${GOPATH}/src/github.com/labstack/echo/recipe/twitter/handler (from $GOPATH)

handler/post.go:8:2: cannot find package "github.com/labstack/echo/recipe/twitter/model" in any of:
    /usr/local/go/src/github.com/labstack/echo/recipe/twitter/model (from $GOROOT)
    ${GOPATH}/src/github.com/labstack/echo/recipe/twitter/model (from $GOPATH)

handler/user.go:9:2: cannot find package "github.com/labstack/echo/recipe/twitter/model" in any of:
    /usr/local/go/src/github.com/labstack/echo/recipe/twitter/model (from $GOROOT)
    ${GOPATH}/src/github.com/labstack/echo/recipe/twitter/model (from $GOPATH)