add Context type

F# doesn't support the GADTs.
So add the ITest interface.
support context
delete unnecessary `end`
pack the active patterns for ITest
Merge pull request #37 from pocketberserker/support-nest

Support nest
remove ToList type class

Use the reflection instead of ToList type class.
Now, Persimmon can handle any n-tuple and non-tuple.
add signature file for NonEmptyList
use FSharpType.IsTuple
start to implement new models
re-impl the test computation expression
add new assertions using new model
re-impl the trap computation expression
rewrite PersimmonTest using new model
re-impl the parameterize computation expression
add Syntax.fs
remove Persimmon.fs
add Writable.fs

eliminate the Writer from new model.
we just use the TextWriter.
add Formatter.fs
improve the SummaryFormatter.normal
change the format of the ProgressFormatter.dot