Remove lowest_layer, add get_lowest_layer (API Change):

fix #1417

* New get_lowest_layer free function works for any object
* New lowest_layer_type trait works for any object
* New examples and documentation on layered streams

API Changes:

* The member function lowest_layer is removed from all
  types provided by the library:

Actions Required:

* Call the free function get_lowest_layer instead of
  member lowest_layer.

tools/user-config.jam

@@ -0,0 +1,12 @@
+# Used on CI
+
+import os ;
+
+local OPENSSL_ROOT = [ os.environ OPENSSL_ROOT ] ;
+
+project
+  : requirements
+    <include>$(OPENSSL_ROOT)/include
+    <variant>debug:<library-path>$(OPENSSL_ROOT)/debug/lib
+    <variant>release:<library-path>$(OPENSSL_ROOT)/lib
+  ;