package: add "superagent" to devDependencies

Also pin the version to >= 0.15.4 and < 0.16.0.

Hat tip to @kaheglar for reporting on the issue tracker.

Closes #1.
Closes #2.
package: use better semver syntax for pinning
History: use * instead of -
v0.0.2
cleanup and refactor to allow for an opts object to be used

Also paving the way for caching the http.Agent on a per-proxy basis.
Planning on using lru-cache for that.
test: add some basic tests for the .proxy() function
index: add support for calling the proxy function directly

For those times when you don't want to extend the prototype...
Still needs tests...
index: include the ":" when generating the cache URI

Not strictly necessary I believe, but feels cleaner to me.
Implement an LRU cache to lower the creation of http.Agent instances
README++
v0.1.0
Add debug
Merge branch 'master' of github.com:TooTallNate/superagent-proxy into TooTallNate-master

Conflicts:
	package.json