Add argparse, argument validation and tests for it

Current arguments allowed are:
- filename: haproxy log filename (with path)
- start: time to begin processing entries from the log file
- delta: time span to process entries up to (starting at 'start')
- command: comma separated list of commands to run on the log
  entries
Update CHANGES.rst
Rename HaproxyLogLine.{parsed,valid}
Add travis support
Update CHANGES.rst
Add coveralls support
Rename README.{md,rst}

.rst is more appropiate as on README restructured text syntax is used
instead of markdown.
Split code in different modules
Add requirements.txt
Run commands passed as arguments

- use eval to run them
- create a main method to ease testing
Add two commands: counter and counter_invalid

With tests, of course.

There's a total_lines attribute too.
Take into account the time range

Make sure (with tests!) that if a start and delta time range is
specified, no log lines will be processed by commands that are
outside of this time range.
Sort log lines

This allows for faster processing on commands that use the time
on the log line for its logic.
Update CHANGES.rst
Comment out by now Python 3.3 on Travis
Make coveralls verbose
Try some settings in .coverage.rc
Maybe without .coverage.rc?