Started to implement Change class
Started documentation and unit tests for Change class
Implemented Change.bells
Implemented Change comparison operators and added missing tests that depended upon them
Updated Makefile clean target to remove build directory
Renamed "ringing-lib" directory to "ringing" to eliminate unpythonic hyphen

This will be necessary in the future if we want to be able to go "from ringing import Row"
Restored package name to "ringing-lib"
More directory renaming: "ringing" -> "src", which fits my usual project layout
Aggregated Row and Change classes into single .pyx file using include statements

I really want to end up with:
 - a single module "ringing" so we can do "from ringing import Row" &c.
 - ideally a single .so file as its heaps more efficient:
    - only have one file to distribute / load
    - link against the Ringing Class Library exactly once, avoiding having to link symbols twice if we ever go static

This commit solves this problem, although potentially introduces some more to think about in the future:
 - include files will not allow classes with circular dependencies (we compile one big file)
 - the single big file has to be compiled by a single thread (it would be better to compile each class separately and aggregate at link stage)