org-babel: Hash based caching of results in buffer

org-babel: Hash based caching of results in buffer

  This commit expands the #+resname: line to include a sha1 hash of
  the contents of the source-code block (including header arguments).
  This hash is saved in raw text in the resname line.  When a source
  block is evaluated it's hash is recalculated and checked against the
  hash in it's results line, if they are equal the current results are
  returned with no recalculation.

  Optional prefix argument when evaluating a source block will force
  re-calculation.

  caching behavior can be inhibited through the use of the :nocache
  header argument.  for global inhibition of caching add :nocache to
  the `org-babel-default-header-args' variable.