Adding doc and cleaning imports

Adding docstring in api_lib.py and resource_base.py.
Cleaning relative imports into absolute imports.
Added external dependencies in README.
Done some small tweaks in api_lib.py and resource_base.py.

Signed-off-by: Hugo Benichi <hugo@midokura.jp>
Merge pull request #7 from midokura/adding_doc

Adding doc and cleaning imports