0.3.0

In this release, in addition to various performance optimization, small bug
fixing, and some internal cleaning-up, the most significant enhancement is the
handling of algebraic systems other than the fermion/boson algebra. Currently,
Clifford algebra and su(2) algebra are supported. Other
algebraic systems should be able to be added with ease.

Also the documentation has been expanded, especially with more examples in the
API reference guide. Also the utilities are added for convenient pickling of
tensors.