Depending on the new version of espower.
Change testing style to eval whole assertion to test two or more args.
Support assertions with two arguments.

In other words, support equal, notEqual, strictEqual, notStrictEqual, deepEqual and notDeepEqual.
Extract assert proxy functions out from closure.
Extract isEspoweredValue function out.
Eliminate PowerAssertContext class to gain composability.
Adjust namings.
Dealing with first argument which is not instrumented.
Replace nested conditional with guard clauses.
Preserve return value if target assertion method returns something.
Seeking for better names.
Extract buildPowerAssertText function out.
Merge pull request #2 from twada/twoargs

Support assertions with two arguments.