Check examples in documentation when running tests

The DocTests.py script will parse a given texinfo file for specially
marked examples, run the ledger command from the example, and check
the result against the example output from the documentation.
Add support to check documentation examples with inline data
Print summary list of failed doc tests if any
Add proper argument parsing to DocTests.py
Add explanation on how to validate documentation examples

by specially marking @smallexample, which will be used by DocTests.py
Merge pull request #240 from afh/pull/DocTests

Validate examples in the texinfo documentation when running tests.