Magic constants

[dcousens]

This pull request does two things.

• It adds the magic protocol constant to the networks object, renaming magicPrefix to messagePrefix (with 1.5.x compatibility added for now)
• It changes estimateFee to use a new function Transaction.prototype.byteLength which is the size calculation functionality extracted out of Transaction.prototype.toBuffer, this saves unnecessary building of a Buffer in estimateFee.

All changes are backwards compatible (excepting 5b4e762), and could be directly applied to 1.5.x via cherry pick.

[dcousens]

@p2pmining and @rubensayshi ACK appreciated if you could :)

[coveralls]

Coverage increased (+0.0%) to 99.17% when pulling 063b036 on magic into 886d38e on master.

[dcousens]

ping @jprichardson

[SerafinTech]

ACK

[rubensayshi]

ACK

[jprichardson]

ACK. But I'm still unsure why you do === undefined instead of the == to leverage type coercian to catch null too...

[dcousens]

@jprichardson what semantic meaning would a null have? In any case, the function will hopefully be gone soon, so this will just be a bike shed :)