Rx PCL target should be changed to portable-win8+net45+wp8+wpa81

[jlaanstra]

Since Rx does not use WCF, the portable target should be updated to portable-win8+net45+wp8+wpa81 (profile259).

Although Rx.NET 2.2.4 adds Universal App support the PCL does not yet include it, or at least not according to the folder in the NuGet package.

Profile259 is the new hotness as discussed in reactiveui/splat#55, reactiveui/ReactiveUI#552 and MvvmCross/MvvmCross#660

/cc @paulcbetts @dsplaisted

[anaisbetts]

@jlaanstra Can you try building a Profile259 PCL and running it on VS2012? If the only people who can use Profile259 are people running VS2013 Update 2 RC, that's a pretty bad look

[jlaanstra]

Quoting myself from reactiveui/splat#55

Visual Studio 2012 on my machine can create a PCL targeting Profile259, but that might be because I have the WP81 SDK.

You can add Profile259 to a PCL targeting Profile78. This also works in VS2012 with and without VS2013 and Update 2 installed so we should be totally fine.

[anaisbetts]

🆒, should've looked that up. Lemme work on a PR for this.

[anaisbetts]

This is fixed in #10