Use psake for building the project

Shortly, large XML-based [Build.proj](https://github.com/HangfireIO/Hangfire/blob/a666bb0fc6ce1f62e94573ab819d0cfe46c4fdaa/build/Build.proj) file was replaced with lightweight and clean [psake-default.ps1](https://github.com/HangfireIO/Hangfire/blob/9d0b6441e0f79d9c5aa51344420ba6f21f2fd353/psake-default.ps1). I can now understand the building pipeline. Besides the switch to the awesome [psake](https://github.com/psake/psake) build automation tool, there are also the following changes:

* Once again, no more large, complicated `build.proj` file, now using clean and maintainable powershell scripts.
* No more `Hangfire.targets` file, we don't need to edit new project files to include it.
* No more weird files under the `build` folder.
* No more deprecated MSBuild-integrated Package Restore, now using [Automatic Package Restore](http://docs.nuget.org/docs/reference/package-restore) in Visual Studio.
* Use ILMerge again instead of ILRepack.
* ILMerging is now like a charm, Dapper assembly merged into Hangfire.SqlServer (closes #266).
* Auto-generation of symbol packages with `*.pdb` files and sources code.
* AppVeyor build version is now sustainable for multiple branches and equal to NuGet packages version.
* Auto-generation of archive file containing all the assemblies for Hangfire.