Math expression documentation

[gianm]

Document the math expression language and how to use it in aggregators or anywhere else it is supported.

[dclim]

This page was started but is unreachable: http://druid.io/docs/0.9.1.1/misc/math-expr.html. Should be polished and linked to docs for 0.9.3

[gianm]

Bumping to 0.10.1