Add support for a wider variety of at rules

Unknown at-rules stil get to be `RuleSet`s but block rules like document/supports/region-style/font-feature-values
are now supported via the AtRuleBlockList class.

Also, all at rules now implement the Sabberworm\CSS\Property\AtRule interface.

MediaQuery has been removed as its functionality is included fully in the AtRuleBlockList class.

Closes #53 as fixed
Update readme to reflect recent changes
Correctly parse negative numbers with no leading 0

closes #54 as fixed
Only use mb_strpos if multibyte support is requested
Strip leading zeros from decimal values
Optimize output of color values

Useful to manipulate colors in integers in PHP code. But once output,
there really is no difference.  And this:

   #fff

is much more space-efficient than this:

   rgb(255,255,255)

(at least for this example, the former is 400% more efficient)
Better way of expressing multiple units

Also will make it easier to extend this check
Fix bug preventing non 'size' numeric values from being parsed correctly

e.g. 10ms was being parsed as Size:10 String:ms, not Size:10ms
Merge pull request #55 from slusarz/strlen

Only use mb_strpos if multibyte support is requested
Merge remote-tracking branch 'slusarz/size'

Closes #57 as fixed
Merge remote-tracking branch 'slusarz/color'

Closes #58 as fixed
Fix documentation comment
Merge remote-tracking branch 'slusarz/unit_fix'
Move all possible units to one place

As constants in the Size class
Do case-insensitive parsing

…as long as identifiers are concerned.

>  All CSS syntax is case-insensitive within the ASCII range (i.e., [a-z] and [A-Z] are equivalent)

(http://www.w3.org/TR/CSS21/syndata.html#characters)
Update README.md

Add note about limited multibyte support
Revert to using simple equals

These identifiers have been parsed case-insensitively so they’re
lower-case either way
Add changelog
Add more units and eliminate some duplicates

see https://developer.mozilla.org/en-US/docs/CSS/length#Units
Fix Size output for locales that use non-period decimal separators

e.g., a locale like pt_PT parses this:

width: 0.15em;

as "0,15".