Use the syntax-propertize mechanism instead of after-change-functions

Apparently fixes #19, but not tested extensively yet.
Check for `syntax-propertize-function' when defining syntax-table

In pre-24 Emacs which lack `syntax-propertize', syntax parsing of
strings and comments can be done by the normal syntax-table mechanism,
which works OK except for the corner cases.
Replace ad-hoc scanning with syntax-propertize-rules
Set syntax table in comint buffer
Add ERT tests to check comment and string syntax.

Hopefully, these will prevent any regression when replacing the ad-hoc
syntax-scanning algorithm with `syntax-propertize` (#19)
Add Makefile targets to run all tests
Fix syntax-propertize rules for empty strings
Fix context tests for syntax-propertize
Merge branch 'syntax-propertize'

Should fix #19.
Fix compile errors when testing from command line
Try to make tests runnable on older Emacs
Update Makefile.in