Update script to v1.5.0

Major update that should fully complete linear-gradient support. Overhauled stop calculation routine as well as added support for repeating-linear-gradient and absolute gradient stop positions (e.g., px, cm, in, etc.)
Update readme to reflect v1.5.0 update
Update advanced-usage-techniques.md
Update changelog.md
Update script to v1.5.1

Squashed a few bugs I saw crop up due to my (failed) experiment with indexOf. D'oh!
Update readme to reflect v1.5.1 update
Update gradient-test-cases.md
Update readme to reflect v1.5.1 update

Changed a sentence I forgot to edit before.
Update readme to reflect v1.5.1 update

One last minor change to a sentence or two. Okay, that should be all for now!
Update to v1.5.1

Removed a stray piece of debugging code I had left behind.
Merge pull request #6 from camartinez1229/master

Update script and documentation to reflect v1.5.1 update