add dllimport property:CallingConvention=CallingConvention.Cdecl

The default callingconvention has always been StdCall.
Starting with the .NET Framework 4, a streamlined interop marshaling architecture provides a significant performance improvement for transitions from managed code to unmanaged code. In earlier versions of the .NET Framework, the marshaling layer detected incorrect platform invoke declarations on 32-bit platforms and automatically fixed the stack.
Merge pull request #5 from lslab/master

add dllimport property:CallingConvention=CallingConvention.Cdecl