Asynchronous HTTP Handler for sitelets
(PageContentAsync and CustomContentAsync and implement IAsyncHttpHandler)

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branch : AsyncSitelets
Merge async with latest

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branch : AsyncSitelets
Async changes

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branch : AsyncSitelets
Fix bug causing unhandled exception when server is restarted and has a dynamic machine key

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branch : AsyncSitelets
Merge with default

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branch : AsyncSitelets
Merged AsyncSitelets.
Fix #208, minimize API changes in Sitelets
Add placeholders into Web.config files in templates for F# 3.1
Merged.
Use latest if-xml
Fix web.config stubs.
Fix #207