Removed Y.cached around _getType, and removed string check

Y.cached(_getType) showed up at about 3-5% in native profiling of new Y.Base()

Digging into it, the work we do to generate the key for cache lookup:

  (if arguments.length check + join + call + in)

Is more than what _getType does:

 (indexOf + concatenate)

Removing it seems to provide benefits, although it's a close call. It'll
definitely help with memory consumption.

Native profiling shows it go down to 1% in new Y.Base().

Relevant benchmarks show improvement across browsers, although it's within the
margin of variance in most cases. That is it's not a slam dunk.

However including the change because when combined with the attrs-perf work, it
shows significant improvements to new Base() times.

Testing
-------

All library tests pass through yogi test, except for the one known failure with
io tests (which will pass once I merge in the latest dev-3.x)

Numbers
-------

THIS COMMIT:

Chrome 25

Publish x 226,567 ops/sec ±19.41%
new Base() x 25,482 ops/sec ±3.19%
Fire With Payload - 0 listeners x 4,717,277 ops/sec ±0.46%
Fire - 0 listeners x 5,210,461 ops/sec ±0.46%

Safari 6

Publish x 341,545 ops/sec ±23.95%
new Base() x 26,091 ops/sec ±0.74%
Fire With Payload - 0 listeners x 3,186,282 ops/sec ±0.60%
Fire - 0 listeners x 3,473,251 ops/sec ±0.71%

FF 19

Publish x 213,839 ops/sec ±6.23%
new Base() x 11,161 ops/sec ±1.91%
Fire With Payload - 0 listeners x 1,695,789 ops/sec ±3.03%
Fire - 0 listeners x 2,196,137 ops/sec ±3.71%

PRIOR COMMIT:

Chrome 25

Publish x 225,817 ops/sec ±23.24%
new Base() x 23,724 ops/sec ±1.91%
Fire With Payload - 0 listeners x 4,639,398 ops/sec ±0.26%
Fire - 0 listeners x 5,147,872 ops/sec ±0.85%

Safari 6

Publish x 342,589 ops/sec ±22.26%
new Base() x 25,457 ops/sec ±0.94%
Fire With Payload - 0 listeners x 3,176,101 ops/sec ±0.51%
Fire - 0 listeners x 3,443,519 ops/sec ±0.54%

FF 19

Publish x 216,064 ops/sec ±6.00%
new Base() x 10,925 ops/sec ±1.65%
Fire With Payload - 0 listeners x 1,658,988 ops/sec ±3.51%
Fire - 0 listeners x 2,194,626 ops/sec ±3.52%

WHEN MERGED WITH ATTRS-PERF BRANCH (WHICH BRINGS OUT THE BLUE IN YOUR EYES):

Chrome 25

This: new Base() x 42,452 ops/sec ±1.58%
Prior: new Base() x 33,146 ops/sec ±0.48%

Safari 6

This: new Base() x 32,689 ops/sec ±0.74%
Prior: new Base() x 29,636 ops/sec ±0.69%

FF 19

This: new Base() x 17,491 ops/sec ±1.18%
Prior: new Base() x 16,490 ops/sec ±1.15%