add a new range adaptor boost::adaptors::ref_unwrapped

The new adaptor is like boost::adaptors::indirected, but for
std::reference_wrapper values (instead of pointer). It calls
.get() on every value and returns the result. It is useful for
range-based iteration of ranges of std::reference_wrapper (or
related) types.

doc/reference/adaptors.qbk

@@ -172,6 +172,7 @@ rng | boost::adaptors::adaptor_generator
 [include adaptors/indirected.qbk]
 [include adaptors/map_keys.qbk]
 [include adaptors/map_values.qbk]
+[include adaptors/ref_unwrapped.qbk]
 [include adaptors/replaced.qbk]
 [include adaptors/replaced_if.qbk]
 [include adaptors/reversed.qbk]

doc/reference/adaptors/ref_unwrapped.qbk

@@ -0,0 +1,32 @@
+[/
+    Copyright 2015 Robin Eckert
+    Distributed under the Boost Software License, Version 1.0.
+    (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+/]
+[section:ref_unwrapped ref_unwrapped]
+
+[table
+    [[Syntax] [Code]]
+    [[Pipe] [`rng | boost::adaptors::ref_unwrapped`]]
+    [[Function] [`boost::adaptors::ref_unwrap(rng)`]]
+]
+
+This adaptor produces a range than applies `.get()` on all values in
+the range. It is useful for iterating ranges of
+`std::reference_wrapper` values or values using similar semantics.
+
+The adaptor is C++11 (and above) only.
+
+* [*Precondition:] The `value_type` of the range has a `.get() const`.
+* [*Postcondition:] For all elements `x` in the returned range, `x` is the result of `y.get()` where `y` is the corresponding element in the original range.
+* [*Range Category:] __single_pass_range__
+* [*Range Return Type:] `boost::unwrap_ref_range<decltype(rng)>`
+* [*Returned Range Category:] The range category of `rng`.
+
+[section:ref_unwrapped_example ref_unwrapped example]
+[import ../../../test/adaptor_test/ref_unwrapped_example.cpp]
+[ref_unwrapped_example]
+[endsect]
+
+This would produce the output `123`.
+[endsect]