Instant Answer: composite units to SI base units

__What does your Instant Answer do?__
Converts composite physical quantities (watts, joules, newtons, etc.) to their SI base unit form (some combination of kg, s, m, A, cd).

__What problem does your Instant Answer solve (Why is it better than organic links)?__
Getting a quick expression for a quantity in si base units makes solving physics problems easier (through unit analysis).

__What is the data source for your Instant Answer? (Provide a link if possible)__
My data source is a text file in share/goodie/si_units/conversions.txt All the information can be verified correct by this Wikipedia article: http://en.wikipedia.org/wiki/International_System_of_Units

__Why did you choose this data source?__
I chose this data source because the list of common composite units is short, and will never change. This eliminates the need to rely on a less reliable source (a website that may or may not exist in the future).

__Are there any other alternative (better) data sources?__
Not that I can think of :(

__What are some example queries that trigger this Instant Answer?__
"newtons in si units", "si units joules", "watts in si base units"

__Which communities will this Instant Answer be especially useful for? (gamers, book lovers, etc)__
students, scientists, physics nerds, engineers.

__Is this Instant Answer connected to a DuckDuckHack Instant Answer idea?__
yep :) https://duck.co/ideas/idea/4725/physical-quantities-in-si-units

__Which existing Instant Answers will this one supercede/overlap with?__
Perhaps the "conversions" instant answer?

__Are you having any problems? Do you need our help with anything?__
YES, I am having a bit of trouble with test files. Every time I try to test my instant answer (prove -Ilib t/SiUnits.t) the values that the console spits out for "got" and "expected" are different. Even though I have not made any changes to any of my files. Simply executing the command again changes my output. See below:
![codio_screencap](https://cloud.githubusercontent.com/assets/9040784/5446464/4127cfc0-848d-11e4-850e-cad18ee789de.png)
This having been said, I have thoroughly tested all possibilities by hand, with no issues.

__Where did you hear about DuckDuckHack? (For first time contributors)__
The link at the bottom of duckduckgo.com :)

__What does the Instant Answer look like? (Provide a screenshot for new or updated Instant Answers)__
![siunits](https://cloud.githubusercontent.com/assets/9040784/5446484/8b660106-848d-11e4-8db9-dc4756e19778.png)

[x] Added metadata and attribution information (https://duck.co/duckduckhack/metadata)
[x] Wrote test file and added to t/ directory (https://duck.co/duckduckhack/goodie_tests)
[x] Verified that instant answer adheres to design guidelines (https://duck.co/duckduckhack/design_styleguide)
[x] Verified that instant answer adheres to code styleguide (https://duck.co/duckduckhack/code_styleguide)
[x] Tested cross-browser compatibility

    Please let us know which browsers/devices you've tested on:
    - Windows 8
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 10

    - Windows 7
        [] Google Chrome
        [] Firefox
        [] Opera
        [] IE 8
        [] IE 9
        [] IE 10

    - Windows XP
        [] IE 7
        [] IE 8

    - Mac OSX
        [x] Google Chrome
        [x] Firefox
        [x] Opera
        [x] Safari

    - iOS (iPhone)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - iOS (iPad)
        [] Safari Mobile
        [] Google Chrome
        [] Opera

    - Android
        [] Firefox
        [] Native Browser
        [] Google Chrome
        [] Opera

Also tested on Arch linux (firefox, and chromium).
I'd probably prefer to get a scalar here, you could `join` the items of the list together