add "trap" computation expression

let! e = trap { f () }

means

let! e =
  try
    f () |> ignore
    failure "..."
  with
    e -> success e

This notation bring us more simplicity.
Merge pull request #25 from pocketberserker/support-property

Support property
add parameter computation expression
add explicit type annotation
use custom operator
rename parameters computation expression
add like TestCaseAttribute of NUnit computation expression
integrate parameterize computation expressions
Merge pull request #20 from pocketberserker/proposal/exn-test

add "trap" computation expression
remove addCase operation
Merge pull request #22 from pocketberserker/topic/parameterized-test

support parameterized test computation expressions
support the test to fail by exn
remove lazy
cover all cases
rename cases of AssertionResult<'T> DU

"Success" and "Failure" are too general.
The name of "Passed" and "Failed" is more focused on the testing.

related to: issue #7