add myself as accumulators maintainer

[SVN r46560]

libs/maintainers.txt

@@ -1,3 +1,4 @@
+accumulators          Eric Niebler <eric -at- boost-consulting.com>
 algorithm/minmax      Herve Bronnimann <hbr -at- poly.edu>
 algorithm/string      Pavol Droba <droba -at- topmail.sk>
 any                   Alexander Nasonov <alexander.nasonov -at- gmail.com>