Add some tests
Fixing issue #1336
Modify test case value

The bugfix #1336 made this test case fails, but, as far I know, the
value for tan( 270deg ) is an absolute Infinity, so I modify the test
case to fit my fix.
Added `define()` bif
Returning "void" `nodes.null`
Fixed: Decimal part of units can be lost when coercing from a string
First stab at adding prefix option to stylus
Added tests for prefixing
Performing a global replace when prefixing
Fixed prefix.extend.with.placeholders test
Moved prefixing to parser
Support multiple selectors in `@extend`. Closes #1418
Made changes per @Panya's suggestion/patch
removed extra this
Fixed bug with the `+=` operator with strings. Closes #743, #1050
Added `--deps` flag to list the dependencies of the compiled file
Fixed bug with hash lookup when a property name is the same as a name of local variable. Closes #1431
Making prefixes work for interpolated selectors
Added prefix grid test case
Implemented built in function prefix-classes