Replace APM-based code with equivalent async/await-based code

As part of a research project at the University of Illinois at Urbana-Champaign, we're developing a refactoring tool that replaces instances of the [APM](http://msdn.microsoft.com/en-us/library/ms228963.aspx) pattern with corresponding [TAP](http://msdn.microsoft.com/en-us/library/hh873175.aspx) method calls and the [async/await](http://msdn.microsoft.com/en-us/library/hh191443.aspx) keywords. I've applied it to this project and found four calls to APM methods.

This pull request replaces those existing calls to APM Begin/End* methods with functionally equivalent TAP constructs and the async/await keywords.

See the following two commits specifically for the actual code-level changes:
* e55150f4a77b2c3b472dcb5db2510633c97ae64d
* 9f1fe35e3e0b2609e0aa0170089dfa10990ce75e

The rest of the pull request is mostly related to NuGet package restore. I've enabled it, otherwise VS 2012 wouldn't allow me to add another dependency that is needed: [Microsoft.Bcl.Async](https://www.nuget.org/packages/Microsoft.Bcl.Async) is a package that adds support for async/await-based programming to older frameworks (Such as WP). As the `packages/` directory is currently under version control, I've tried to keep it as clean as possible (i.e. I've removed all non-'sl4-windowsphone71' directories).

Are you interested in merging this pull request? If not, please let me know why, and I'll try and improve the pull request with your comments in mind.

Thanks for your time,
David Hartveld