Remove the dependency of lxml

[hupili]

Just helped a new user on getting SNSAPI up for the first run on his fresh Ubuntu system. I find that installing lxml is no so straightforward. Users should have python-dev and libxslt installed. New users (to Linux and Python) may have difficulty figuring it out.

The solution:

• Only leave pure Python module in requirements.txt. Guarantee a smooth dependency installation experience via pip.
• Dynamically load plugin from a config file. Experienced users can choose to enable extra plugins. custom_config.py can be a good place to do this.

[hupili]

Following is one way to gracefully manage it:

• a requirements.txt only includes the minimal requirements that would get normal platforms work.
• a requirements.txt.extra includes extra dependencies required by some platforms, e.g. lxml.

Besides, load requirements.txt in setup.py instead of hardcoding.

This makes sure casual users can do a pip install snsapi smoothly and get hands on in a few minutes.