Extract formatter

[twada]

Trying to extract power-assert-formatter out.

empower module does not provide default formatter from now on.
power-assert module will compose assert object/function and formatter using empower function.

TODO

• Delete power-assert-formatter related files
• Change empower function signiture
• Fake formatter for unit testing
• Create power-assert-formatter module
• Recover test coverage
• Make formatter a function, not object
• Do not provide bind for old browsers
• Refactoring

[coveralls]

Coverage decreased (-11.84%) when pulling b6e1145 on extract_formatter into ee8dbce on master.

[coveralls]

Coverage decreased (-0.73%) when pulling 21d1772 on extract_formatter into ee8dbce on master.

[coveralls]

Coverage decreased (-3.37%) when pulling 82971d7 on extract_formatter into ee8dbce on master.

[coveralls]

Coverage decreased (-0.07%) when pulling b8af83d on extract_formatter into ee8dbce on master.