Fix docs for the get command, add option -n to string the final newline

Using -n provides a cleaner way to put e.g. a password into a command's option

Add a get CLI command

Add the get command, which returns a key's value immediately to stdout (or a realm as JSON)

This is useful where a value is needed on the command line (where an environment value won't work,
because it's not already in the environment

For example,

some command opt=`envy get realm/key`   ### envy exec realm some command opt=$key won't work

Fix the usage statement

No other changes to the functionality

Fix issues 3-5 [breaking changes]

#3 envy.New no longer requires a directory; use envy.NewWithDirectory to provide one explicitly
#4 read and dump are renamed write and read to be more intuitive when used
#5 read -q unquotes JSON embedded in values

Add commands to read or write a realm to JSON

Merge pull request #2 from matt4biz/json

Add read/dump subcommands, add some app UTs, and a little code cleanup

Initial version of the program

Merge pull request #1 from matt4biz/initial

Merge the initial development branch