Some performance tweaks for testing.

Assert.* methods take a `msg` argument, which is preformatted.  This
means that we end up taking the hit of formatting arrays/etc even
if the assert passes.  Change to formatting internally to avoid
this.

Formatting the expression stack seems to take longer than we'd like.
Capturing just the frame doesn't work for some reason, so make
it an optional flag.

Try to pretty print stacks a little bit.  Common regions of the
stacks should be removed as well at some point.
Disable logging of traces by default.
Merge pull request #50 from fegin/master

Make dot expr as an Expr
assert methods should have a common interface.
Fix `le` method.
Merge pull request #48 from spartan-array/perf-tweaks

Some performance tweaks for testing.
finish kmeans
add kmeans file
use KMeans of sklearn in test
remove kmean exmaple; add reducer to new_centers and new_counts array
use spartan.zeros instead of using map to reset new_centers and new_counts
fix kmeans, use ndarray instead of expr.zeros
return centers in KMean
Merge pull request #52 from EasonLiao/sklearn_kmeans

Sklearn kmeans
Rename MapResult to LocalKernelResult, move to core.

MapResult is a confusing name, and we had no documentation for what
it is for (mainly for deferring blocking on futures).  Added
documentation and updated references.
Fix imports.
Merge pull request #55 from spartan-array/refactor-map-result

Rename MapResult to LocalKernelResult, move to core.
Make transpose expr zero-copy.
Merge remote-tracking branch 'upstream/master'