Bug fixed
Add flag for enabling/disabling expression caching.

This also checks the global optimization flag.

Additionally, reduce some logging and catch import errors
when serializing.
delete isomap
update
Some bug fixed and improvement from OSDI experiments.
1. Add two more way to assign tiles, serpentine and static.
2. In order to support static assigne, we need extents can be sorted.
3. Add a configuration to config the maximum zeromq sockets.
   The zeromq default setting is too small for more than 256 workers.
4. The original count_nonzero can't count sparse correctly. Fixed this bug.
Typo
Merge pull request #143 from spartan-array/cache_optimization

Add flag for enabling/disabling expression caching.
Merge pull request #148 from EasonLiao/app_refine

some optimization from scalability tests
Merge pull request #149 from fegin/optimization

Some bug fixed and improvement from OSDI experiments.
Merge branch 'master' of github.com:spartan-array/spartan

Conflicts:
	spartan/examples/cf/item_recommender.py