Memoize string constants

old:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  8.670000   0.080000   8.750000 (  8.736402)
      user     system      total        real
instrumented  8.480000   0.080000   8.560000 (  8.544077)
      user     system      total        real
instrumented  8.470000   0.070000   8.540000 (  8.533670)

new:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  7.520000   0.080000   7.600000 (  7.594280)
      user     system      total        real
instrumented  7.440000   0.080000   7.520000 (  7.513975)
      user     system      total        real
instrumented  7.710000   0.080000   7.790000 (  7.781426)
Speed up generate_guid

old:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  7.580000   0.080000   7.660000 (  7.647277)
      user     system      total        real
instrumented  7.620000   0.080000   7.700000 (  7.695582)
      user     system      total        real
instrumented  7.780000   0.070000   7.850000 (  7.849560)

new:
      user     system      total        real
instrumented  7.270000   0.070000   7.340000 (  7.348110)
      user     system      total        real
instrumented  7.390000   0.070000   7.460000 (  7.471058)
      user     system      total        real
instrumented  7.420000   0.080000   7.500000 (  7.491155)
Less calls to Kernel#nil?

new:
~/dev/agent_prof [master] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  7.160000   0.080000   7.240000 (  7.227999)
      user     system      total        real
instrumented  7.240000   0.080000   7.320000 (  7.318296)
      user     system      total        real
instrumented  7.330000   0.080000   7.410000 (  7.402006)

old:
~/dev/agent_prof [master] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  7.240000   0.080000   7.320000 (  7.318199)
      user     system      total        real
instrumented  7.300000   0.080000   7.380000 (  7.377251)
      user     system      total        real
instrumented  7.430000   0.080000   7.510000 (  7.497654)
Optimize for empty hash

cuts down on method invocation, though not much of a speedup
Don't dup apdex_t and convert to float everytime we use it.
Don't even call store_sample_for_developer_mode if we're not in dev mode

It's surprisingly expensive

old:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented 18.910000   0.180000  19.090000 ( 19.077663)
      user     system      total        real
instrumented 18.970000   0.200000  19.170000 ( 19.136990)
      user     system      total        real
instrumented 18.900000   0.180000  19.080000 ( 19.071671)

new:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented 18.890000   0.200000  19.090000 ( 19.068841)
      user     system      total        real
instrumented 18.810000   0.180000  18.990000 ( 18.973000)
      user     system      total        real
instrumented 18.790000   0.170000  18.960000 ( 18.950816)
Minimize calls to String#split

old:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  7.110000   0.080000   7.190000 (  7.177364)
      user     system      total        real
instrumented  7.150000   0.080000   7.230000 (  7.218395)
      user     system      total        real
instrumented  7.040000   0.080000   7.120000 (  7.111533)

new:
~/dev/agent_prof [master*] $ for i in {1..3};do  ruby nr_bm.rb ; done
      user     system      total        real
instrumented  7.000000   0.080000   7.080000 (  7.076323)
      user     system      total        real
instrumented  7.080000   0.080000   7.160000 (  7.148165)
      user     system      total        real
instrumented  6.990000   0.080000   7.070000 (  7.054725)
Merge branch 'dev' into samg/perf

Conflicts:
	lib/new_relic/transaction_sample.rb
Remove old rails 3 view instrumentation that didn't work
Got a very preliminary version of the instrumenation working
Don't cause errors for actions that don't render a standard template
Merge branch 'dev' into seve/rails3_view_instrumentation
version and gemspec for 3.3.4.alpha
Rails 3.2 view instrumenation
Rails 3.0 view instrumentation
Extract some rails 3 view instrumentation logic into helper module.
Better handling of Rails 3.0 file rendering
Merge branch 'dev' into seve/rails3_view_instrumentation
Merge branch 'dev' into samg/perf
Merge branch 'seve/rails3_view_instrumentation' into dev