added WP8 compiler symbol which incorporates awaitable async methods into the build
Rewrote UrlEncodeStrict

It didn't handle % characters in the original string (unless they were
followed by a not Unreserved Character).
Added some unit tests as requested
Fixed PercentEncode StringExtension

This was also doing some special handling for \b\r\n and ignored the
issue with hex characters 00..0F (was encoding (some of) them as single
character strings %0 for example).
Touching up the unit tests
Touching up the unit tests for AppVeyor
Test touchup for appveyor again
Merge pull request #597 from chris-grabcad/master

Rewrote UrlEncodeStrict
Merge pull request #596 from bicycle-nerd/wp8-compiler-symbol

Added WP8 compiler symbol which includes awaitable async methods