Simplify grammar

[JSAbrahams]

• Change assign symbol from '<-' to ':='
• Change inheritance from 'isa' to ':'
• Remove 'raises' keyword, replace all instances with 'raise'

Summary

It looks better I think, and is likely more familiar to most. The only other language which uses '<-' as far as I'm aware is R.

Added Tests

Tests rewritten to deal with symbol change.