Corrects the handling of function mappers, addressing jnr/jnr-ffi#13.
LibraryLoader now uses its functionMappers member in line with the
design. The contract of FunctionMapper.mapFunctionName is made
explicit and a bug fixed in CompositeFunctionMapper. Tests are added
to LibraryLoaderTest.
Merge pull request #22 from jeff5/master

Corrects the handling of function mappers, addressing jnr/jnr-ffi#13.