Added doctest and method description in context.py

Added doctest for method textFile and description for methods _initialize_context and _ensure_initialized in context.py

Author: Jyotiska NK <jyotiska123@gmail.com>

Closes #187 from jyotiska/pyspark_context and squashes the following commits:

356f945 [Jyotiska NK] Added doctest for textFile method in context.py
5b23686 [Jyotiska NK] Updated context.py with method descriptions

(cherry picked from commit 9cff1dd25abc5e848720d853172ed42e35376fd0)
Signed-off-by: Matei Zaharia <matei@databricks.com>