Utilities for cache implementation.

Added functions to read/write repos list in cache file.
Implemented cache system.

A bit of refactoring for cache implementation.
Implemented cache system.

At the first execution of `helm-github-stars`, list of repositories is
fetched from github and saved into a cache file, default location:
~/.emacs.d/hgs-cache.
Minor improvements.

Added helm-github-stars to fetch repositories befor opening helm
interface.
Fixed some typo
Fixed test functions name (-test prefix)
Eldoc text
Added missing interactive form.
Updated version to 1.0.1