build-0006
_splat.c: improve documentation + Fragment.mix

* improve docstring of Fragment.mix and Fragment.normalize
* change signature: Fragment.mix(offset, start, length) to allow
  more flexible mixing
* define minmax macro to limit a numerical value between 2 bonds
splat/data.py: improve Fragment docstring
_splat.c: add Fragment.resize and remove set_duration

* remove set_duration, now the duration attribute is read-only
* add Fragment.resize method which can be used to grow or shrink
  the fragment
* change frag_resize so it can grow or shrink (using a length in
  samples)
* add frag_grow macro to resize only to grow as frag_resize used
  to do, and use it in place of frag_resize
splat/data.py: remove Fragment.resize + tweak docstrings

* remove splat.Fragment.resize as it's now implemented in C in
  _splat.Fragment.resize
* tidy-up docstrings in n2s and s2n
_splat.c: improve documentation + add 2014 to copyright
splat/data.py, gen.py: tweak docstrings + add 2014 to copyright
doc: review and improve + add automodule directives
doc/conf.py: add 2014 to copyright