initial commit for the elm logger

A basic implementation of ELM, a wannabe ELMAH. Currently the logs are written to a dictionary and are displayed in plain text

![elmloggerinitial](https://cloud.githubusercontent.com/assets/2818455/4650805/c8578f66-5496-11e4-9e85-dc8d51da1d1a.png)

This field seems terribly confusing, as it represents the key associated with the unique request identifier in `context.Items`, but not the request identifier itself. `_requestIdentifierKey` would be a better name, IMO.

Anyway, once https://github.com/aspnet/HttpAbstractions/issues/117 is implemented, this field might become unnecessary.