Included LastModified in response headers

[dmalanij]

This is a solution for #201.

Implemented logic to keep record of the timestamp when a response content was generated and use it as the Last-Modified response header.
The implementation is quite basic and tried to reduce the impact, however it has a breaking change on the signature of ApplyCacheHeaders method.
And it could be improved, depending how you'd like to expose this, by:

• Using an additional boolean property in the CacheOutputAttribute to control whether the header is included or not
• Trying to avoid inline serialization / deserialization of the timestamp (by either changing the restriction in the Get<> of the IApiOutputCache, or using a wrapper object which could also help with the Replace 3 cache calls with ICacheItem #41)

Additionally, I can include in the Pull Request some changes on the Demo application to support the CORS request that the scenario will probably require (and some other tweaks on it to make it easier for any developer to use it)

[filipw]

thanks!