Prior to setting the PKG_CONFIG_PATH, as below, pip install cryptography failed with Symbol not found: _ffi_type_double on my machine running Mac OS X 10.8.5. I am not sure how commonly the error occurs for other users.
brew install libffi
# Note the version-specific path below.
PKG_CONFIG_PATH=/usr/local/Cellar/libffi/3.0.13/lib/pkgconfig/ pip install cryptography