.so versioning fixes; added installation settings

- The .so versioning was not right: fixed it.
- Use GNUInstallDirs to get installation locations
- Add install() comments so that make package/make install should (more
  or less) work.
No longer include MathJax; use .html suffix

- Use system MathJax, if we can find it
- If not found, fall back to CDN-hosted MathJax
- Files now use .html suffix instead of .xhtml
- README.md gets included as main API doc page
Random: thread-safety; simpler seeding

The code before was definitely not thread-safe.  This new code gives
each thread its own RNG object.  ERIS_RNG_SEED now sets the seed for
the first thread to need a seed; the second gets ERIS_RNG_SEED+1, the
third gets ERIS_RNG_SEED+2, etc.

When ERIS_RNG_SEED is now get, we now rely on the system's random
source via std::random_device instead of the rather messy tick finding
code that was there before in cycle.h.
FindMathJax.cmake: locating system MathJax

Since MathJax is no longer included, this is needed to find the
system's MathJax.