Minor - added header-only flag in conan

See header-only guidelines: 
http://conanio.readthedocs.io/en/latest/howtos/header_only.html?highlight=header%20only
Its borderline cosmetic, but it does have a purpose.

conanfile.py

@@ -14,3 +14,6 @@ class CatchConan(ConanFile):
 
     def package(self):
         self.copy(pattern="catch.hpp", src="single_include", dst="include")
+    
+    def package_id(self):
+            self.info.header_only()