Make AdditiveGroup extend Group like it should.
Merge remote-tracking branch 'upstream/master' into conversions

Conflicts:
	src/main/scala/spire/algebra/Group.scala
Merge remote-tracking branch 'upstream/master' into conversions
Fix bug causing low priority Signed to return -1 instead of 0.
Add missing ConvertableTo implicits.
get rid of anonymous classes
remove private val
reenable Complex[A]#hashCode override
reenable complex test and update comments
make Signed[A]#signum abstract
Get Spire basically working with 2.10.0-M3

Currently ScalaTest dependency is disabled, so I can't run the tests.
At this point all I know is that it compiles and some of the benchmarks
run successfully.

Specialization has been disabled due to SI-5788 breaking a lot of our
recursive code. Hopefully it will get fixed; if not, we'll need to
restructure some things.
Update to 2.10.0-M4, fixing macros, tags, etc.

There are still a ton of warnings, and specialization is still broken, but at
least the code compiles and seems to work so far.
Add sealed to correct exhaustiveness warnings
Resolve the rest of the exhaustiveness warnings.

Spire is now warning-free in 2.10.0-M4!
Get tests running (and passing) in 2.10.0-M4
add sbt helper script
replace Manifest with ClassTag, and fix fallout
ignore .lib left behind by ./sbt
First pass at implementing macros for typeclass operators in Spire.

This is a gigantic commit. It has some other necessary restructuring mixed in
as well, which probably should have been pulled out into its own commit. This
was mostly just to demonstrate that the approch works, and to try to get the
2.10.0 branch a bit more up-to-date.
Forbid zero denominators in Rational#build.