Adding first prototype extension and benchmarks.

A basic integer extension is now in ext/bson/native.c. Along with this
is a rake task for showing pure ruby vs with extension comparisons via
"rake bench".

The default rake task now cleans before running specs.

First run with basic integer extension:

Cleaning out native extensions...
Benchmarking pure Ruby...
BSON is using the pure Ruby implementation.
user     system      total        real
String#to_bson -->  1.180000   0.020000   1.200000 (1.204498)
Integer#to_bson ->  0.640000   0.030000   0.670000 (0.662678)
Compiling native extensions...
Benchmarking with C extensions...
user     system      total        real
String#to_bson -->  0.600000   0.000000   0.600000 (0.602383)
Integer#to_bson ->  0.130000   0.000000   0.130000 (0.130826)
Dont commit the bundle
Updating ignored files