index typo
add fromBN convenience function

It is very often the case that you might want to create a private key from a
big number.
add bitcoinjs ecdsa test fixture

...and test the "valid" part of the test fixture
minor typo
add TODOs to test the invalid fixtures
change .sign and .verify to return this

One of the primary goals of fullnode as a library is internal consistency, and
part of the philosophy of the objects is that the instance (protoype) methods
modify the object and then return this. This allows you to change the functions
together, e.g. ecdsa.sign().verify().sign(). Although this may or may not be
useful in particular for signing and verifying, in order to retain the internal
consistency and be like the other objects, ECDSA should behave that way.
also test that k is calculated correctly

...against the bitcoinjs test fixture.
static methods should start with @