[Xml] Remove bad assert

The assert would be triggered if the document started with
a closing tag, which is invalid XML but can be typed by users.
[Ide] Don't error on missing gravatars

If an email address doesn't have a gravatar associated
with it, this is normal, not an error.
[C] Fix backtick expansion issue on MacOS

MacOS' sh has a built in implementation of echo that ignores the -n
option, causing it to get passed to the compiler instead.
[Toolbox] Minor cleanup, add .NET 4 support
[Xml] Keep track of parser location when backtracking
[Xml] Improve some error locations

If an error has no explicit position, place the marker before
the current character, not after it.
[CSharp] Don't deadlock by starving task scheduler

Every time the caret moved the C# completion code was creating
a new task that blocked on fetching the shared resolver. Huge
numbers of these could become created over time and completely
starve the task scheduler, causing a deadlock.

Use continuations instead.

Fixes BXC18138 - XamarinStudio Crashes when editing Razor view