Fix for bad Date#toJSON polyfills.

[bryanforbes]

Fixes #95.

[bryanforbes]

!~ was an attempt to make the feature test suggested in #95 a bit shorter. I suppose it's just obfuscating what is actually going on. I've pushed a change to the branch that's more obvious. I've also stored -62198755200000 and '-000001' in variables.

I'm also wondering if the indexOf check could be changed to !(new Date(negDate).toISOString().indexOf(yearStr)) instead of (new Date(negDate).toISOString().indexOf(yearStr) === -1) since I believe '-000001' should be at the beginning of the string. Can '-000001' be anywhere else in the string?

[kriskowal]

This looks good to me.