Update version numbers to 0.8

[brson]

No description provided.

[thestinger]

@alexcrichton: ideally we would set this in the Makefile and have it based on git describe except in a release tarball

if you check out a tag like 0.7, the git describe output will simply be 0.7 and then it will tack on the # of commits since the last tag + the commit id when you go past it

[alexcrichton]

That sounds like a good idea to me! It looks like the usage falls into three categories:

• README.md - not sure if this could get automatically updated (I don't think it should either though)
• *.po files - these are generated, right?
• src/libX/X.rs - this would involve env!() to work in cfg, although that'd probably make things too annoying to compile by hand.