Improve C++14 support:

* Make functions constexpr.
* Make functions noexcept where appropriate.
* Add test case for the above.

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 <p><b>Table of Contents</b></p>
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 <dt><span class="section"><a href="index.html#boost_integer.overview">Overview</a></span></dt>
 <dt><span class="section"><a href="boost_integer/traits.html">Integer Traits</a></span></dt>
 <dt><span class="section"><a href="boost_integer/integer.html">Integer Type Selection</a></span></dt>
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 <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
-<td align="left"><p><small>Last revised: June 01, 2014 at 19:57:36 GMT</small></p></td>
+<td align="left"><p><small>Last revised: March 28, 2017 at 17:56:42 GMT</small></p></td>
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