Almost working excel reports.

* Not sure why balances don't change after 2013-09-30.
* Still want to put a cmd-line interface on this.
* Should clean up code. Rename things,
  extract excel-suitable formatting and values on
  a per-ccy basis.
Minor cleanup & improvement of excel-output formatting.

Problem with balances not changing after 2013-09-30 was
just shortcoming in list of transaction data.
Temp checkin - revert this with better code.

This now can put transactions into excel report - they're
treated like a sub-account - just nested below the parent
account.

Still pretty ugly code, and may well have broken stuff that
used to work.
More colour coding of excel reports.

More code to clean up.
Minor improvements - added transaction list report.
Add "chart of accounts" to excel output.

Use namedtuple a bit more.
Can still generate the original text-based report.

At least for the one test case I bothered putting together.
Hooked up new excel-generating functionality to command-line args.

Example invocation:
  ledger.py foo.transactions --generate-excel-report foo.xls  --dates 2013-06-30 2013-09-30 2013-12-31 2014-03-31 2013-06-30
Improvments to documentation.
Bug fix so --print-register example still runs.