Replace const keywords with var to support ES5 builds

[mohammadrafigh]

No description provided.

[holgerd77]

Hi, thanks for the PR, but we are just in the process of using babel for the various ethereumjs libraries to create ES5 distributions, so this probably won't get merged.

I'll prepare a PR for this library soon.

[mohammadrafigh]

ok cool and thanks.

[holgerd77]

Hi @mohammadrafigh, I directly added the PR #37. Could you do a short review (optimally with the review function if available to you, eventually from the "Files changed" tab)?

[coveralls]

Coverage remained the same at 86.62% when pulling c6a57f9 on mohammadrafigh:master into 01382b1 on ethereumjs:master.