Use ctypes find_library to discover libcrypto better

I ran into an issue trying to use PyChef in Docker containers:
```
  File "/.virtualenvs/sys/local/lib/python2.7/site-packages/chef/__init__.py", line 5, in <module>
    from chef.api import ChefAPI, autoconfigure
  File "/.virtualenvs/sys/local/lib/python2.7/site-packages/chef/api.py", line 18, in <module>
    from chef.rsa import Key
  File "/.virtualenvs/sys/local/lib/python2.7/site-packages/chef/rsa.py", line 9, in <module>
    _eay = CDLL('libcrypto.so')
  File "/usr/lib/python2.7/ctypes/__init__.py", line 365, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libcrypto.so: cannot open shared object file: No such file or directory
```
Replicating the code in `rsa.py` that tries to load `libcrypto.so` I get the same error:

```
# python
Python 2.7.3 (default, Mar 13 2014, 11:03:55)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import ctypes
>>> ctypes.CDLL('libcrypto.so')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python2.7/ctypes/__init__.py", line 365, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: libcrypto.so: cannot open shared object file: No such file or directory
```

Basically, there is no `libcrypto.so` library and there is no symlink for `libcrypto.so.1.0.0` to `libcrypto.so`, so `ctypes.CDLL('libcrypto.so')` fails. Looking at the python executable being used, it is linked to `libcrypto.so.1.0.0`, not `libcrypto.so` which is probably why CDLL is failing:

```
# ldd `which python`
	linux-vdso.so.1 =>  (0x00007fff223f0000)
	libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f74b1a0d000)
	libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f74b1809000)
	libutil.so.1 => /lib/x86_64-linux-gnu/libutil.so.1 (0x00007f74b1605000)
	libssl.so.1.0.0 => /lib/x86_64-linux-gnu/libssl.so.1.0.0 (0x00007f74b13a7000)
	libcrypto.so.1.0.0 => /lib/x86_64-linux-gnu/libcrypto.so.1.0.0 (0x00007f74b0fcc000)
	libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f74b0db4000)
	libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f74b0ab8000)
	libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f74b06f8000)
	libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f74b04e1000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f74b1c30000)
```

Looking at [the ctypes documentation](https://docs.python.org/2/library/ctypes.html#finding-shared-libraries) there is a function called `find_library` that helps find linked libraries in a smart way. Using `find_library` cytpes is able to detect the libcrypto library linked with python and use it:

```
# python
Python 2.7.3 (default, Mar 13 2014, 11:03:55)
[GCC 4.7.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from ctypes import util
>>> util.find_library('crypto')
'libcrypto.so.1.0.0'
```

Using this method allows `rsa.py` to find the correct linked version of libcrypto and does not require os-level symlinking shenanygans. :)