Add Short Mining Section

* High-level overview of mining; some details may be not-quite-correct.
  Corrections welcome.
Add SOMEDAY for p2pool in mining section (thanks @instagibbs!)
Updating Mining Section Based On Feedback From @instagibbs and @saivann

This is the "haste makes waste" update.  I should've known better than
to try to write the Mining section quickly.  :-)

Over 100 net lines were added, making the following changes:

* Changed introduction to define solo/pool miners by their goals rather
  than whether they're peers. Suggested by @instagibbs (thanks!)

* (No substantive changes to the solo mining text.)

* Removed extra ASICs from solo mining illustration.

* Generalized how pool miners get work to allow for different methods
  described later.

* Removed extra mining pool and ASICs from pool mining illustration.
  Changed "block template" (still used in solo illustration) to
  "prototype block". Based on feedback from @saivann and a specific
  suggestion by @instagibbs (thanks!)

* Give `getwork`, `getblocktemplate`, and Stratum each their own
  subsubsection describing them in overview-level detail using parallel
  structure for easy comparison.
Merge remote-tracking branch 'saivann/dev' into mining

Conflicts:
	en/developer-guide.md
Mining Section Tweaks Suggested By @saivann (thanks!)

* Use the term "variance" for payment frequency

* Mention that getblocktemplate also allows miners to remove some txes
Merge pull request #66 from harding/mining

Add Short Mining Section