Micro Optimizations, plus cleaning up ATTRS cache a little.

Key wins seem to be:

a) Avoiding the delete operator, and just set to undefined/false
b) Using cfg by reference where possible (as opposed to getting/setting in state again)

As a result of a) had to update an attribute test for the _getAttrCfg protected method, since
we leave more internal meta-properties in the attr config hash (instanced of deleting them).

attribute, base, widget, dd and app tests all pass.

NOTE: We can get a bit more of a performance boost, once we clean up app's id handling to avoid
the addAttr overide (we need to update one of of state's internal meta-properties [lazy],
for that, which we could avoid). Without this, there was a failing model-rest-sync test
(discussed with Eric).

This commit (Chrome 23):

MyBaseCore with 20 varied attributes (using perf. best practices) x 24,336 ops/sec ±1.70% (93 runs sampled)
MyBaseCore with 20 varied attributes x 9,748 ops/sec ±5.56% (64 runs sampled)
MyBaseCore with 10 simple value attributes x 51,166 ops/sec ±1.91% (93 runs sampled)
MyBaseCore x 159,111 ops/sec ±1.58% (94 runs sampled)
BaseCore x 164,114 ops/sec ±1.69% (94 runs sampled)

Last commit (ATTRS cache):

MyBaseCore with 20 varied attributes (using perf. best practices) x 20,857 ops/sec ±1.94% (94 runs sampled)
MyBaseCore with 20 varied attributes x 9,384 ops/sec ±4.12% (71 runs sampled)
MyBaseCore with 10 simple value attributes x 36,140 ops/sec ±2.03% (92 runs sampled)
MyBaseCore x 78,936 ops/sec ±3.45% (89 runs sampled)
BaseCore x 81,023 ops/sec ±2.64% (90 runs sampled)

3.8.0:

MyBaseCore with 20 varied attributes (using perf. best practices) x 11,805 ops/sec ±1.89% (89 runs sampled)
MyBaseCore with 20 varied attributes x 7,039 ops/sec ±3.74% (73 runs sampled)
MyBaseCore with 10 simple value attributes x 21,069 ops/sec ±1.66% (93 runs sampled)
MyBaseCore x 60,417 ops/sec ±2.23% (90 runs sampled)
BaseCore x 61,683 ops/sec ±0.92% (86 runs sampled)