'release-prep' task, and wheels

• new: added 'release-prep' task
• new: added --skip-root to 'check', and checking './*.py' too
• new: 'dist' task automatically creates wheels if possible
• chg: better handling of 'build --docs'
• chg: added help for task parameters (closes #4)
• chg: warn about missing Sphinx docs (when '--docs' is provided)
• fix: get src package list for 'check' from 'project.packages'
• fix: use 'which' to look for 'py.test' binary (closes #2)