Added expression binding logic

[Stevenic]

Fixes #1818

Description

Class memory scope wasn't expanding expressions properties.

Specific Changes

• Added logic to expand expression properties.
• Added missing DialogClassMemoryScope.
• Fixed case for scope enums and added doc comments.

Testing

Updated unit tests to test property expansion.

[stevengum]

I have some misgivings about the name *ClassMemoryScope, but it otherwise looks fine... Tests would be nice 😄

[coveralls]

Pull Request Test Coverage Report for Build 127935

• 30 of 39 (76.92%) changed or added relevant lines in 6 files are covered.
• 9 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.1%) to 79.848%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
libraries/botbuilder-dialogs/src/memory/scopes/dialogClassMemoryScope.ts	5	14	35.71%

Files with Coverage Reduction	New Missed Lines	%
libraries/botframework-streaming/src/webSocket/webSocketTransport.ts	2	80.65%
libraries/botframework-streaming/src/webSocket/webSocketServer.ts	7	66.67%

Totals
Change from base Build 127853:	-0.1%
Covered Lines:	12462
Relevant Lines:	14891

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💛 - Coveralls