add item-based collaborative filtering.
add test for item-based cf
add sort into expr
change svm code to allow optimization
a much simpler and more scalable qr decomposition
Merge pull request #136 from fegin/optimization

Some updates for optimization.
Merge pull request #139 from MaggieQi/master

add sort into expr
Merge pull request #142 from fegin/optimization

Optimization bugs fixed
some fixes
Merge pull request #135 from EasonLiao/cf

add item-based collaborative filtering.
fix expr
Merge pull request #140 from EasonLiao/fix_qr

a much simpler and more scalable qr decomposition
Add flag for enabling/disabling expression caching.

This also checks the global optimization flag.

Additionally, reduce some logging and catch import errors
when serializing.
Add some statistical operation benchmarks.
Start a benchmark suite for fusion-capable expressions.
Merge pull request #146 from spartan-array/fusion_tests

Start a benchmark suite for fusion-capable expressions.
Merge pull request #145 from spartan-array/statistic_operations

Add some statistical operation benchmarks.
small rpc optimization
make random forest better
change tile.pyx to allow constructing object dtype array.
Merge pull request #147 from EasonLiao/rf_opt

make random forest better