Fix client test environment

[frankrousseau]

I made several improvements to make the addition of tests easier:

• Small fix of account store that requires the window object to run properly ;
• Add some utils to mock easily the action dispatcher ;
• Add Eslint configuration to lint test files.

Be aware that tests will require Node 4 to run properly.

[frankrousseau]

@m4dz I performed the fix you requested. Thank you for the review!