Adding PCL support.

This commit includes a new assembly that will can be deployed on
.net4.5,windows mobile, droid, and iOS. The client also takes advantage
of async/await support as well as optional parameters, simplifying the
total number of available methods in the client.
Implements async version of 1.x methods.

Initial implementation of v1.x API calls in the client. Still needs unit
tests for each, as well as parsing of error messages from endpoints.
Allow retrieval/updates of client's server settings.
Extracting client to Base Class

* Extract core components to base class, allowing creation of an "Admin"
client.
* Adding "Admin" client to support account-wide API requests.
Adding support for triggers and stats APIs
Adding Inbound Trigger Management
Adding admin client support for sender signature management.
Implement the missing stats calls.
Cause the send test to use the recipient address instead of the sender.
Stubbing out client integration tests.
Adding Unit Tests for AdminClient Method Calls.
"Fixing" overzealous DKIM AdminClient test.
Adding additional Integration Tests for the 2.0 client.
Adding test coverage for other parts of the client.
Requiring configuration variables to be set before running tests.
Cleaning up compilation warnings.

- ReplayTo on .Net's Message class must continue to be accessed/mapped
as consumers of the client may not have stopped using this property.
- LinqBridge specifies an ExtensionAttribute, but one is already
available elsewhere.
- Some test fixture setups run synchronously, awaiting a completed task
suppresses the warning.
Eliminate app.config in Postmark.Tests in prep for CI builds.
Adding XML documentation to the PostmarkAdminClient
Adding lookups for testing variables.
Adding appveyor.yml