Improvements to Logentries appender and documentation

Did a substantial clean-up and improvement of the documentation.
Added more explanatory information about appender configuration, and added more prescriptive guidance.
Changed the appender to use the new unified ConfigurationManager for hosting-agnostic configuration, making it useful in more application environments.
Changed credential configuration names to better conform with established conventions for appSettings (but retained support for the older names for compatibility).
Removed unused method SubstituteAppSetting().
Added VS project and solution, allowing the appender to be compiled and referenced.
Updated library version number to 2.4.0.
Updated the documentation to reflect all the above changes.