Move pk_info structure to its own file

This is in preparation to adding support for opaque keys in the pk layer. The
header files are for use as follows:

* pk.h for users of the pk interface.
* pk_info.h for implementers of opaque key engines.
* pk_internal.h only inside libmbedcrypto to implement built-in pk types.
diff --git a/visualc/VS2010/mbedTLS.vcxproj b/visualc/VS2010/mbedTLS.vcxproj
index f13f83c..1a55eaa 100644
--- a/visualc/VS2010/mbedTLS.vcxproj
+++ b/visualc/VS2010/mbedTLS.vcxproj
@@ -191,6 +191,7 @@
     <ClInclude Include="..\..\include\mbedtls\padlock.h" />

     <ClInclude Include="..\..\include\mbedtls\pem.h" />

     <ClInclude Include="..\..\include\mbedtls\pk.h" />

+    <ClInclude Include="..\..\include\mbedtls\pk_info.h" />

     <ClInclude Include="..\..\include\mbedtls\pk_internal.h" />

     <ClInclude Include="..\..\include\mbedtls\pkcs11.h" />

     <ClInclude Include="..\..\include\mbedtls\pkcs12.h" />