This version takes to add the include path to the target definition for
cmake 2.8.11 and later.  Also the documentation sticks to the existing
convention of using cmake variables ${PROJ4_LIBRARIES} and
${PROJ4_INCLUDE_DIRS}.  However, the namespace variables are still being
included.

Here's the roll-out plan

(0) Version 4.9.x: The target is proj and PROJ4_LIBRARIES is set to
this.

(1) Version 5.0.x: Two targets, proj and PROJ4::proj, are defined;
PROJ4_LIBRARIES = proj.

(2) In a year or two: Two targets, proj and PROJ4::proj, are defined;
PROJ4_LIBRARIES = PROJ4::proj.

(3) With a change in the library which breaks backwards compatibility:
The target is PROJ4::proj and PROJ4_LIBRARIES = PROJ4::proj.

* replace SVG figure by PNG version,
* rewrite maths using :math:,
* put references in the main reference section,
* reformat references for consistency,
* put references in alphabetical order,
* use USGS URL for Snyder (1987).

calculations with something more helpful to the beginning user.

Central conic projection implemented as 'ccon'.

The equations on the documentation page for geodesics were plaintext
formatting and also contained a typo. This patch updates the formatting
to use the sphinx supported MathJax syntax and corrects the typo.

Better plots for documentation