Added additional conditions to allocation tests to confirm the created BS* isn't null
Reduced level of detail in header file docblocks.

The header file used to document all permitted return values for conversion
functions. This level of detail will inhibit future development of the library
if clients come to depend on this behaviour. I've reduced the level of detail
simply to itemise the return values and their causes, and added a warning not
to trust the byte stream if a conversion fails.
Restored check to CK_FORK for base64 tests now debugging is complete
Changes to bs_hex source and test.

Modified testcase to make sure we attempt to load all possible hex characters.
Previously we only used a small range of lowercase characters and digits.

Added an assertion that the created byte stream is the correct size.

Fixed an arithmetic error which meant it wasn't.
Untidied the Makefile

Partially reverts commit 670f59d. The changes made by that commit had the
unanticipated effect of breaking the CFLAGS setting for compilation. This meant
that the -g flag fell off and debugging became rather more tricky than
expected.

Restores individual _CCFLAGS and _LDADD entries for test programs.
Updates to base64 tests

Rewrote some pointer arithmetic in test_load until GCC stopped raising
insignificant warnings about pointer type coercion.

Added a test to check that BS_INVALID is returned if the input string is an
invalid length.