render timestamp in its own method so callers can override its format
don't even try to get hostname if it won't be used
factor out render_name so callers can reformat the name+pid
Merge pull request #1: Refactor timestamp and name formatting into their own methods

I made the following significant changes during merging:

- Add docstrings & mention subclassing in README.md
- Don't call os.getpid() when the result won't be used
- Don't call render_*() methods when results won't be used

See pull request #1 on GitHub:
  https://github.com/xolox/python-coloredlogs/pull/1