Make #fetch and #fetch_multi emit ActiveSupport::Notifications for instrumentation of any sort.

This makes IDC emit [`ActiveSupport::Notifications`](http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html) for fetches so we can start tracking hits and misses in statsd and other stuff eventually. 

On ze branch:
```
oration ~/C/identity_cache  (instrumentation_hooks) ➜  bx rake benchmark:cpu
/opt/boxen/rbenv/versions/1.9.3-p448/bin/ruby ./performance/cpu.rb
Database not ready for performance testing, generating records
Rehearsal ----------------------------------------------------------
FindRunner:              1.260000   0.100000   1.360000 (  1.759094)
FetchMissRunner:        21.820000   1.270000  23.090000 (139.928938)
FetchHitRunner:         35.350000   2.390000  37.740000 (158.807096)
DoubleFetchHitRunner:   34.660000   2.400000  37.060000 (155.994105)
DoubleFetchMissRunner:  30.040000   2.130000  32.170000 (150.531126)
----------------------------------------------- total: 131.420000sec

                             user     system      total        real
FindRunner:              1.180000   0.110000   1.290000 (  1.688685)
FetchMissRunner:        21.350000   1.250000  22.600000 (139.036593)
FetchHitRunner:         32.350000   2.260000  34.610000 (152.959066)
DoubleFetchHitRunner:   34.360000   2.400000  36.760000 (155.403047)
DoubleFetchMissRunner:  29.790000   2.120000  31.910000 (149.987077)
```

On ze master:
```
oration ~/C/identity_cache  (master) ➜  bx rake benchmark:cpu
/opt/boxen/rbenv/versions/1.9.3-p448/bin/ruby ./performance/cpu.rb
Rehearsal ----------------------------------------------------------
FindRunner:              1.180000   0.110000   1.290000 (  1.694002)
FetchMissRunner:        22.030000   1.280000  23.310000 (140.561444)
FetchHitRunner:         35.520000   2.410000  37.930000 (159.102155)
DoubleFetchHitRunner:   36.220000   2.490000  38.710000 (158.796237)
DoubleFetchMissRunner:  31.360000   2.180000  33.540000 (152.982849)
----------------------------------------------- total: 134.780000sec

                             user     system      total        real
FindRunner:              1.240000   0.120000   1.360000 (  1.791787)
FetchMissRunner:        22.070000   1.300000  23.370000 (140.411133)
FetchHitRunner:         32.430000   2.300000  34.730000 (153.066971)
DoubleFetchHitRunner:   34.540000   2.430000  36.970000 (155.922840)
DoubleFetchMissRunner:  29.870000   2.150000  32.020000 (150.509368)
```

@camilo @fbogsany thoughts?