Bump version to 0.1.5.0
Add a sumVector function
Update changelog
Add a pairwise summation implementation

It's oddly slow (!?).
Bump version to 0.1.5.1
Improve summation docs
Added tag 0.1.5.1 for changeset 5b10f37b6178
Try to fix build
Bump version to 0.1.5.2
Added tag 0.1.5.2 for changeset d902a472f4df
Add flag to compile with SSE2 on i386

The default for the flag is ON, which is a reasonable assumption for the
vast majority of modern machines.

We require SSE2 for tests to avoid problems with excess precision. This
fixes test failures for Sum on i386.
Remove flag for setting -fsse2