implement the -cpuprofile flag

Noticed the `-cpuprofile` flag was there, but the profiling was missing.

Added profiling as described in [Profiling Go Programs](http://blog.golang.org/profiling-go-programs).

Example output for the load operation (with leveldb) of `30kmoviesdata.nt`:

![cpu](https://cloud.githubusercontent.com/assets/53705/3541482/b5359388-084c-11e4-9005-9d9ccdfdf976.jpg)

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