PBKDF2 support for OpenSSL backend
pbkdf2 docs
test coverage, other changes
documentation improvements for KDF
remove length check (which cffi handles)
rename PBKDF2 to PBKDF2HMAC, address many other review comments
finish PBKDF2HMAC rename, more docs
okay this time really finish the rename. Up example iterations to 100k
update docs with more detailed info re: PBKDF2 usage
Merge branch 'master' into pbkdf2-openssl

* master:
  Fuck, fix
  C is not a good programming language
  This needs to be first
  Doh, reorder
  This type is not defined either of course
  Whoops, backwards
  Fixed #521 -- work on systems with no ec header at all
add PBKDF2HMAC to changelog
simplify pbkdf2_hmac_supported in openssl backend
Merge branch 'master' into pbkdf2-openssl

* master:
  Add this to the changelog

Conflicts:
	docs/changelog.rst
add some unicode checks for salt on init and key_material on derive
rename test files
some alternate language
commoncrypto PBKDF2 support
update commoncrypto pbkdf2 with new naming
update changelog
doc updates based on review