More convenient way to call async methods on hub

Added two additional overloads for CallAsync method:
1) To call hub's methods which return Task
2) To call hub's methods which return Task<T>
All the magic is handled by SignalR.
Merge pull request #1 from Hinidu/call-task-methods

More convenient way to call async methods on hub
moved nuspec file to the project folder
prepared for appveyor
finished implementation of appveyor build process and nuget auto deployment
fixed appveyor.yml
removed duplicated assemblyconfiguration attribute
all warnings are now treated as errors
removed sample projects from solution and updated references
added some unittests
fixed duplicate key exception
implemented all necessary tests
changed async call to sync call
fixed copyright char displayproblem
build process is adding nuget deps automatically to nuspec