Update Workbook proxy

[timhall]

Tasks:

• Add AfterEach to teardown workbook after each test
• Add instance to BeforeEach and AfterEach methods to allow passing in workbook helper instance
• Separate from core into extensions (WBProxy and Scenario)
• Use copy of workbook in case it's already open or unexpected things happen
• Explore using standard WBProxy in other IWBProxy implementations. (lots of overlap currently)

BeforeEach and AfterEach goal:

Dim Proxy As New WBProxy
' Initial setup/mapping...

Suite.BeforeEach "Setup", Instance:=Proxy
Suite.AfterEach "Teardown", Instance:=Proxy

Causes an issue with current arguments implementation, since ParamArray cannot be used with other Optional variables (Instance), but ByRef with BeforeEach and AfterEach is a goal, so ParamArray might not be useful anyways.

[timhall]

This seems much more likely with the evented approach used in v2 and the Context object proposed in #23 so this is seeming much more straightforward. Basically, it will open / copy the workbook before each test, pass it in as .Context("workbook"), and close the workbook without saving after each test.

Dim Suite As New TestSuite

Dim Fixture As New WorkbookFixture
Fixture.Open "path/to/workbook.xlsm"
Fixture.ListenTo Suite

With Suite.Test("should open workbook")
  .IsEqual .Context("workbook").Name, "workbook"
End With