Use priority to sort exercises
Add an example of use of priority
Merge pull request #18 from OKU1987/oku/priority

Use priority to sort exercises
Revise examples of use of priority
Merge pull request #19 from OKU1987/oku/priority

Revise examples of use of priority
Add tests for Store classes.
Some minor changes to the Store class.
Fix Store#[]=.

The transaction opened in Store#[]= doesn't exit normally,
because of 'return self'
Merge pull request #20 from chiro/test-store

Fix Store#[]=.
Add status 'initial submit'.
Add account management.