Use built-in STL now that we don't use a libstdc++ tag file.
Move the main page for the Doxygen documentation out of hummstrummengine.hpp into a separate file in the doc/ subdirectory.
Modify the overview section in the Doxygen documentation to clarify the intent of the documentation and change it to an introduction section.
Remove unnecessary description of the documentation structure, and replace it with more useful information about the purpose of the documentation.
Add an exception guarantee directive to our Doxygen configuration.  Now, we can document the exception guarantee with the @exceptguarantee tag.
Add additional resources section on the mainpage of the Doxygen documentation.  It contains links to project resources and other documentation.
Add a page for conventions used within the documentation, link to it from the mainpage, and add a description of how exception guarantees should be documented.
Remove unused header and footer templates, and CSS stylesheet.
Hide detail namespaces from our user-facing API documentation.
Change Doxygen style color from blue to green to match our website better.
Add sections on invariant, move semantics, and pre-/postcondition documentation in the conventions page.
Merge branch doxygen-cleanup into unstable. doxygen-cleanup contains changes to make our Doxygen API documentation easier to use, such as a page to describe the conventions that we use in the documentation, and a rewrite of the main page.