separate out offsets and timeouts

[thediveo]

This is a (late) follow-up to #392 and separates control over offset, timeout and polling intervals from the multitude of expectation-related function signatures. In consequence, test writers can stick solely to the baseline of Expect, Eventually, Consistently in the future:

• WithOffset(offset int) on (async) assertions sets the offset, so ExpectWithOffset(666, ...).To(FooBar()) can now be written as Expect(...).WithOffset(666).To(FooBar()). The same applies to Eventually and Consistently.
• async assertions additionally feature WithTimeout(interval time.Duration) and WithPolling(interval time.Duration). Here, I opted for a type-safe interface and not repeating "Interval" in the method names, as the function parameter names already carry this meaning.
• existing interfaces are kept unmodified.
• internal/ tests are updated to use WithOffset, WithTimeout and WithPolling.
• godoc comments have been updated.

[onsi]

This looks great - thanks so much!