v0.27 - Measure CPU Time not Wall Clock Time

Changes:
- Spiral's `seconds_spent` field now counts seconds of CPU time spent processing the spiral, not wall-clock time (the former is a more faithful reflection of processing complexity when compared with others on the same system).
- Marked `sxbp_resize_spiral()` as deprecated from the Public library interface.
- Amend directory and instructions to easily facilitate out-of-source builds.

Tiny file format version bugfix

Fixes a tiny weeny bug (#170) where the minimum supported file format version was incorrectly specified as **v0.25.0** when it is in fact **v0.26.0**.

Breaking file format release

**BREAKING RELEASE!**

Improvements:

- 16-bit version identifiers in library and file format
- 32-bit friendly code
- storing of time spent generating a spiral (and the accuracy of this figure) in file

Doc updates

- Added a neat little Doxygen badge
- Cleaned up the code style a little bit
- Added better notation of in/out parameters in documentation

Update copyright years and other doc embellishments.

Add doxygen documentation to source control

More liberally-licensed, now with Doxygen documentation

- Software license changed to MPL-2.0
- Doxygen documentation added

PNG Output bugfix

This bugfix version fixes an issue where PNG output could never be enabled (caused by some symbols which were missed in the renaming).

Renamed Macro bugfix

- fixes a bug where a macro was not renamed in source (it was renamed in the build scripts).

Rename

This version update renames the library in source from libsaxbospiral to libsxbp