Auto-generated documentation with TSDocs

[holgerd77]

Documentation of this library should be auto-generated with TypeDoc, this PR ethereumjs/ethereumjs-util#170 can serve as a reference (see also technical docs on documentation).

For this the manual documentation comments in the README (https://github.com/ethereumjs/ethereumjs-account) should be integrated in the code along the way. The code examples should either kept integrated or separated, whichever makes more sense respectively is more practical.

Depends on #27

[holgerd77]

@s1na Have labelled this "good first issue" and "help wanted", this doesn't mean that you can't take on this if you want which would also make sense since you did the preparatory TypeScript work.