requirements: Drop some optional requirements

Move some non pure python requirements (namely lxml and paramiko)
from pip-requires to test-requires.  Document it in README.rst.

Motivations:

- Make this pip-installable with pypy
- Less distribution package requirements (deb/rpm/etc) in case
  a user do not actually need the functionality

Note: while paramiko itself is pure python, it requires pycrypto.
Note: msgpack has pure python fallback implementation.

Separating ryu into smaller packages would be a better alternative.
However, it would involve a lot more work than this workaround.

Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

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