Adding new sub-command "field" to "droplet"

To make scripting of pontoon commands (such as chaining them in bash scripts)
a lot easier, a new sub-command was added to the DropletCommand that allows
querying/outputting the value of a single field of a droplet's meta-information.
fixed pep8 issues
Extended description on running tests when submitting PRs

[skip ci]
Merge pull request #19 from nureineide/master

Adding new sub-command "field" to "droplet"