Remove sync calls

Most of Async.RunSynchronously calls have been removed.
Fix #262.

There are two places I don't know yet how to remove them:
 - https://github.com/dungpa/PowerTools/blob/removing-sync-calls/src/FSharpVSPowerTools.Logic/XmlDocFilter.fs#L50

 - https://github.com/dungpa/PowerTools/blob/removing-sync-calls/src/FSharpVSPowerTools.Logic/NavigateToItem.fs#L192

Any idea is appreciated.

This change affects most functionalities. Please check carefully.