Merge pull request chainer#406 from chainer/update-to-v0.6

Update the version to v0.6

Merge pull request chainer#354 from prabhatnagarajan/grammar_nit

nit on grammar of a comment:

Merge pull request chainer#289 from chainer/update-version-v0.4

Update to v0.4

Merge pull request chainer#199 from chainer/muupan-patch-4

Update to v0.3.0

Merge pull request chainer#106 from chainer/muupan-patch-1

Update to v0.2.0

Merge pull request chainer#57 from muupan/pcl-prioritized-replay

Use Prioritized Replay for PCL