Bump Version 0.5.1
Bump Version 0.5.1

ChangeLog:

- PyClient: Make some attributes (`servers`, `comp_threshold`)
  accessible from Python code
- Return specific error code.

douban/libmc,lihuanshuai/libmc,mckelvin/libmc,mckelvin/libmc,mckelvin/libmc,douban/libmc,mckelvin/libmc,douban/libmc,douban/libmc,mckelvin/libmc,lihuanshuai/libmc,douban/libmc,lihuanshuai/libmc