Update Build Requirements and Instructions

Updates our build requirements based on our new testing environment.

- `Cython` required version is bumped to 0.19 due to relative imports from `_shared`.
- `Matplotlib` and `networkx` have been made optional since our tests now pass without them.
- The min Python version is bumped to 2.6 since that is the lowest we test against.
- The min `Scipy` version is dropped to 0.9 since we are testing against that version on Python 3.2

Dependencies are stored in `requirements.txt` and read into `setup.py`.  In the future, changes to versions will have to be made in `requirements.txt` and `DEPENDS.txt` only.

Here's the traceback on Cython 0.17:

```python
Traceback (most recent call last):
  File "setup.py", line 151, in <module>
    cmdclass={'build_py': build_py},
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/core.py", line 152, in setup
    config = configuration()
  File "setup.py", line 52, in configuration
    config.add_subpackage('skimage')
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/misc_util.py", line 1002, in add_subpackage
    caller_level = 2)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/misc_util.py", line 971, in get_subpackage
    caller_level = caller_level + 1)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/misc_util.py", line 908, in _get_configuration_from_setup_py
    config = setup_module.configuration(*args)
  File "skimage/setup.py", line 16, in configuration
    config.add_subpackage('filter')
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/misc_util.py", line 1002, in add_subpackage
    caller_level = 2)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/misc_util.py", line 971, in get_subpackage
    caller_level = caller_level + 1)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/numpy/distutils/misc_util.py", line 908, in _get_configuration_from_setup_py
    config = setup_module.configuration(*args)
  File "skimage/filter/setup.py", line 18, in configuration
    cython(['rank/generic_cy.pyx'], working_path=base_path)
  File "/home/silvester/workspace/scikit-image/skimage/_build.py", line 41, in cython
    cythonize(pyxfile)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 603, in cythonize
    aliases=aliases)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 546, in create_extension_list
    kwds = deps.distutils_info(file, aliases, base).values
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 457, in distutils_info
    return (self.transitive_merge(filename, self.distutils_info0, DistutilsInfo.merge)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 467, in transitive_merge
    node, extract, merge, seen, {}, self.cimported_files)[0]
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 478, in transitive_merge_helper
    for next in outgoing(node):
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Utils.py", line 29, in wrapper
    res = cache[args] = f(self, *args)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 406, in cimported_files
    pxd_file = self.find_pxd(module, filename)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Utils.py", line 29, in wrapper
    res = cache[args] = f(self, *args)
  File "/home/silvester/anaconda/envs/py26test/lib/python2.6/site-packages/Cython/Build/Dependencies.py", line 388, in find_pxd
    raise NotImplementedError("New relative imports.")
NotImplementedError: New relative imports.
```