Fix Windows to look in Assembly Directory for 32bit dll.

Commit 96eb5e3 introduced the use of `DefaultDllImportSearchPathsAttribute`
to add extra security when loading native libraries. This
works fine when the executable that is running is in the
same directory as the `LibZipSharp.dll` and the `libzip.dll`.
However when the executable has a different `WorkingDirectory`
as the `LibZipSharp.dll` this then causes the following
error.
System.DllNotFoundException: Unable to load DLL 'libzip'
This is because with the new attribute only Safe paths are searched.
So when we run this under MSBuild.exe for example, the `Working`
directory will definately NOT contain the 32bit `libzip.dll`.
The fix to to always set the location of the `libzip.dll` via
`SetDllDirectory` for both 32bit and 64bit processes.
Also bump to 1.0.18

Author: dellis1972

LibZipSharp.props

@@ -1,5 +1,5 @@
 <ProjectDefaultTargets="Build"ToolsVersion="4.0"xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
- <_LibZipSharpNugetVersion>1.0.17</_LibZipSharpNugetVersion>
+ <_LibZipSharpNugetVersion>1.0.18</_LibZipSharpNugetVersion>
  </PropertyGroup>
 </Project>

Native.cs

@@ -415,9 +415,7 @@ static Native ()
 {
 if(Environment.OSVersion.Platform==PlatformID.Win32NT){
 stringexecutingDirectory=System.IO.Path.GetDirectoryName(typeof(Native).Assembly.Location);
-if(Environment.Is64BitProcess){
-SetDllDirectory(System.IO.Path.Combine(executingDirectory,"lib64"));
-}
+SetDllDirectory(Environment.Is64BitProcess?System.IO.Path.Combine(executingDirectory,"lib64"):executingDirectory);
 }
 }
 }