Allow using = in the end of an env var's value and some improvements

Summary of changes in this PR:

 * Improve comments a little bit

 * Improve docs for setting up development environment

 * Allow using = in the end of an env var's value

    Environment variables that contain an equal sign (=) in the end are now matched. For example:

    BASE64_STRING=t5urur0ck5==

    This change breaks the usage of env-set. If a given value contains any space, it **has** to be enclosed by either single or double quotes.

    This is ensured by asserting the equality of both the number of arguments provided by the shell (i.e. bash) and the number of matches found.

    Test and usage example were updated accordingly.