particular, I removed the enumerator over blocks in favor of a simple
get_blocks() function that just returns a std::vector of all the blocks.
I also removed the requires clauses on the block and key accessor functions
and instead made a request for a non-existent key/block result in a non-fatal
exception.  This way users can let the config reader perform a more natural
role in config file validation (by catching this exception and acting accordingly).

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403077