Overhaul docs and makefile

[Ichimonji10]

Overhaul the underlying structure of the documentation.

• Expand docs/index.rst with information taken from README.rst,
  docs/installation.rst and docs/getting_started.rst.
• Make README.rst a link pointing to docs/index.rst.
• Drop docs/installation.rst and docs/getting_started.rst.

Why has this been done?

• Making README.rst a link means that there is now a single, authoritative
  source for documentation. Formerly, README.rst and the various docs/*
  files contained much duplicate information.
• Merging docs/installation.rst and docs/getting_started.rst into
  docs/index.rst is simply sensible. The documents are short, and it is
  reasonable for their information to be contained in one page.

Update the makefile with a more robust, expansive and consistently named set of
rules.

• The rules are more robust because they use && instead of ; for compound
  commands.
• The rules are more expansive because a new rule has been added
  (docs-doctest) and the doctests are run when the test rule is evaulated.
• The rules are more consistently named because the prefix "docs-" is used to
  denote all rules defined in docs/Makefile.

Fix broken doctests. It is now easy to test this because make test runs both
normal unit tests and doctests.

[coveralls]

Coverage remained the same when pulling 0bd0119 on Ichimonji10:docs-makefile into bf2cd25 on omaciel:master.

[omaciel]

ACK