use Gradle 1.7 wrapper instead of bundled

The bundled Gradle in AS 0.2.7 is a nightly 1.8 build that doesn't seem to work well with AS
add Message classes

since Message's fields are a
subset of Post's, Post now extends
Message.

There is a single retrieveMessages() method
in AppDotNetClient, but it involves pagination
parameters, so we should probably consider
pagination as it applies to posts, etc. too before
finalizing this. It was mostly just for testing.
add AbstractPost base for Post and Message
move id field to Annotatable
extract id string creation for use with other retrieve multiple endpoints
add machineOnly constructor param for posts and messages
add Count for integer data

Allows for primitive integer as the data field in a response envelope while maintaining the IAppDotNetObject hierarchy
initial Channel support
initial Message support
prevent empty annotation serialization