Change package name, and move PPX executable

[mrmurphy]

Three things in this commit! I initially tried to split it into couple of commits, but changes ended up overlapping so I threw them all into one PR.

• Change the package name to decco
• Move the ppx executable to the root level and called it ppx. (see reasoning below)
• Change the ppx executable to use bash instead of Node for perffffffffffffffff.

Notes on the moving of the PPX:

The current state of bucklescript ppxs is rather confusing, each having its own executable path. Every time the user wants to install a PPX, they have to check the readme to see what particular location they have to point to for the executable. With bs-let I moved the executable to the root level and named it ppx, so that when the user is installing, all they have to do is add package_name/ppx to the ppx flags to get it installed and working. If all bucklescript PPXs followed this pattern, users wouldn't need to refer to the readme at all after the first few times.

What do you think? @ryb73

[ryb73]

Looks good! Just a couple comments.

[mrmurphy]

@ryb73 thanks for the review! I addressed your comments. Want to take another look?