Fix transformRe (fixes #1658)

Allow optional units in the transformation matrix, the presence of which
causes maxBounds settings not to work in (at least) older versions of Firefox.

Also update the regex to match on numbers per W3 spec: "A number is either an
<integer> or zero or more decimal digits followed by a dot (.) followed by one
or more decimal digits." (from CSS Values and Units Module Level 3)
Merge pull request #1659 from karlnack/fix-ff-maxbounds

Update L.PosAnimation._transformRe to account for units (fixes #1658)