Passing cfg through to _fireAttrChange, since we already have a reference

Improves performance for set(), and hence also new Base().

Chrome is pretty much a wash, probably because it's doing something smart
with the function hops.

Testing
-------

All library tests pass:
[Total]: Passed: 10730 Failed: 0 Total: 10819 (ignored 89) (8 minutes, 17 seconds)

Numbers
-------

Chrome 25:

This Commit

Base x 20,433 ops/sec ±1.99% (88 runs sampled)
MyBase x 20,767 ops/sec ±0.65% (97 runs sampled)
MyBase with 10 simple value attributes x 15,255 ops/sec ±1.26% (96 runs sampled)
MyBase with 10 simple value attributes - set x 108,367 ops/sec ±0.83% (91 runs sampled)

Prior Commit

Base x 21,380 ops/sec ±2.20% (91 runs sampled)
MyBase x 20,438 ops/sec ±2.42% (93 runs sampled)
MyBase with 10 simple value attributes x 15,052 ops/sec ±1.62% (92 runs sampled)
MyBase with 10 simple value attributes - set x 103,998 ops/sec ±0.93% (94 runs sampled)

Safari 6:

This Commit

Base x 22,983 ops/sec ±0.46% (68 runs sampled)
MyBase x 21,138 ops/sec ±0.67% (66 runs sampled)
MyBase with 10 simple value attributes x 17,763 ops/sec ±0.74% (66 runs sampled)
MyBase with 10 simple value attributes - set x 88,091 ops/sec ±0.74% (64 runs sampled)

Prior Commit

Base x 18,937 ops/sec ±0.74% (66 runs sampled)
MyBase x 18,192 ops/sec ±0.74% (66 runs sampled)
MyBase with 10 simple value attributes x 15,284 ops/sec ±0.64% (64 runs sampled)
MyBase with 10 simple value attributes - set x 71,233 ops/sec ±0.73% (66 runs sampled)

FF 19:

This Commit

Base x 11,360 ops/sec ±1.31% (86 runs sampled)
MyBase x 11,104 ops/sec ±1.14% (85 runs sampled)
MyBase with 10 simple value attributes x 9,437 ops/sec ±1.30% (90 runs sampled)
MyBase with 10 simple value attributes - set x 52,091 ops/sec ±0.91% (86 runs sampled)

Prior Commit

Base x 10,955 ops/sec ±1.45% (86 runs sampled)
MyBase x 10,762 ops/sec ±1.21% (86 runs sampled)
MyBase with 10 simple value attributes x 9,018 ops/sec ±1.52% (90 runs sampled)
MyBase with 10 simple value attributes - set x 48,555 ops/sec ±0.74% (87 runs sampled)