HDNode / BIP32 Neutering

[dcousens]

This pull request adds BIP32 style neutering of nodes by re-creating a node with all the private key information stripped.
This is useful for when you want to distinctly store or transport a HDnode without exporting it publicly then re-importing it as can be seen in af47371.

[dcousens]

There are arguments for and against allowing the isPrivate flag on toBase58.
If we were going for a removal of the flag in 2.0.0, then an assert should be included for the transition IMHO.

[coveralls]

Coverage increased (+0.0%) when pulling af47371 on dcousens:hdnode into a8c6f52 on bitcoinjs:master.

[coveralls]

Coverage increased (+0.0%) when pulling 903f412 on dcousens:hdnode into 04bcbad on bitcoinjs:master.

[weilu]

What we currently have:

• private hdnode exports private: hd.toBase58()
• public hdnode exports public: hd.toBase58()
• private hdnode exports public: hd.toBase58(false)
• public hdnode exports private: hd.toBase58(true) //error

With this PR + we drop the isPrivate flag on toBase58, we will have:

• private hdnode exports private: hd.toBase58()
• public hdnode exports public: hd.toBase58()
• private hdnode exports public: hd.neutered().toBase58()
• public hdnode exports private: // impossible because API is such

I agree that the latter is better because it gets rid of a boolean arg and elegantly makes the error case impossible. My vote goes to deprecating the boolean flag on toBase58 with this PR, bump minor version after this is merged. With 2.0.0, we remove the boolean arg, until then the error path is still possible.

[kyledrake]

+1 with @weilu's deprecation proposal.

[dcousens]

Rebased on HEAD and added deprecation warning.

[coveralls]

Coverage increased (+0.01%) when pulling 6b42949 on dcousens:hdnode into 5888ca5 on bitcoinjs:master.