Make Installation section self-contained and clearer.

[sietse]

• Dulwich requires a C compiler at installation time; this is now mentioned in "Installation" instead of further down in "Dependencies". This leaves "Dependencies" naming the runtime dependencies, and only those.
• "Installation" now mentions where to get easy_install/setuptools, for those that don't have it.
• "Installation" now explains the difference/advantage of manual install: that one runs Hg-Git from a repository clone rather than from a static package.
• "Dependencies" now mentions what Dulwich is, for the curious.

[durin42]

Thanks!