Redirect whitepaper/download links; fix grammar/word usage

Link to /whitepaper and /downloads instead of directly linking to
binaries.  This change also includes various grammatical fixes and word
substitutions (for example, "coin age" rather than "coinage" to
correspond with the language used in the whitepaper).
Merge pull request #308 from confit/master

Redirect whitepaper/download links; fix grammar/word usage