Mock performance.now(), fixes #24

[insidewhy]

No description provided.

[coveralls]

Pull Request Test Coverage Report for Build 45

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 43:	0.0%
Covered Lines:	18
Relevant Lines:	18

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💛 - Coveralls

[insidewhy]

It might be nice to provide an API to advance the performance.now() time beyond the now() time in order to test certain conditions for users of window.performance.now, but this PR is probably a good start.

[hustcc]

It might be nice to provide an API to advance the performance.now() time beyond the now() time in order to test certain conditions for users of window.performance.now, but this PR is probably a good start.

Also, document of readme should be update.