General cleanup

This pull request renames `D` to `d` for private key notation.  This is as per the [SEC](http://www.secg.org/download/aid-780/sec1-v2.pdf) paper, and is more consistent with most literature on ECDSA.

It also fixes a capitalization typo in the `Script` test fixtures, and attempts to use the `forEach(func() { it(desc, func() {}) })` testing structure when testing fixtures.
The only module not using this style now is the hash functions in `crypto`; and it is undecided whether it is necessary to expand these vectors or not.