RF minor: doc-str, repr, imports
DOC
NF: class SecStr() to manage sensitive strings, eg passwords

- mainly: raise error if try to get the string value, eg in a print.
  requires asking explicitly for property `.str`
- experimental: zero out memory upon del(), or clear()
- limitations:
  - not sure if 32 vs 64 bit arch is handled correctly
  - RsaKeys passphrase does not use a SecStr, should do
RF: some smoothing in genrsa

- also, not yet used: hack getpass module, embed SecStr
RF: less verbose exception classes
BF SecStr.zero

- left over from incomplete refactor
RF: use SecStr for RSA pwd

- use a thread to zero the pwd after RSA decrypt it; AES decrypt
  can take minutes for a big file, no need to keep pwd in memory
- class PyStringObject may not be right for 64 bit
  had to fudge the offset (-4);
  sizeof() each element suggests should be 36 not 40
TEST add .travis.yml

- not sure that the .yml config is correct...