Remove non-portable CrashLogInformation from PCL

Phones and desktop apps have fairly different crash log header
info to send. Construct the header string in the CrashHandler
instead of trying generalize it in CrashLogInformation.

This avoids pointless empty fields in the crash log headers.  For example:

```
Package: WpfSample
Version: 0.0.9.0
OS: Microsoft Windows NT 6.3.9600.0
Windows Phone: 
Manufacturer: 
Model: 
Date: 2013-12-18T21:39:45.4771296Z
Product-ID: 
```

becomes:

```
Package: WpfSample
Version: 0.0.9.0
OS: Microsoft Windows NT 6.3.9600.0
Date: 2013-12-18T21:39:45.4771296Z
```

And this:

```
Package: PhoneSample
Version: 1.0.2.0
OS:
Windows Phone: 7.10.7720
Manufacturer: Microsoft
Model: XDeviceEmulator
Date: 2012-10-05T16:00:42.7820000Z
Product-ID: {040a7121-e4cf-44cf-b5a3-ja2bfe424188}
```

becomes this:

```
Package: PhoneSample
Version: 1.0.2.0
Windows Phone: 7.10.7720
Manufacturer: Microsoft
Model: XDeviceEmulator
Date: 2012-10-05T16:00:42.7820000Z
Product-ID: {040a7121-e4cf-44cf-b5a3-ja2bfe424188}
```