Fix and test bug in fmpq_abs.

fmpq_abs used to only touch the numerator of the target fmpq, causing
errors in virtually any invocation. This was missed because there used to
not be a test file for fmpq_abs.

Remedy both issues.
tweak create_doc.c
fixes to documentation

Also bail out in create_doc.c if parse exception is encountered.
Merge remote branch 'tom/gsoc-doc' into trunk
fix bug in is_squarefree
Merge remote branch 'tom/is_squarefree' into trunk
make is_squarefree an fmpz_poly method; slightly simplify it
document fmpq_poly_reverse