Fix byte-compile warnings

I got following byte-compile errors when I byte-compiled.

```
Compiling file /home/syohei/tmp/gomi/helm-proc/helm-proc.el at Wed Mar 26 18:23:11 2014
Entering directory `/home/syohei/tmp/gomi/helm-proc/'

In helm-proc-candidates:
helm-proc.el:90:9:Warning: reference to free variable `for'
helm-proc.el:90:13:Warning: reference to free variable `candidate'
helm-proc.el:90:23:Warning: reference to free variable `in'
helm-proc.el:91:9:Warning: reference to free variable `collect'

In helm-proc-system-pgrep:
helm-proc.el:95:9:Warning: reference to free variable `for'
helm-proc.el:95:13:Warning: reference to free variable `result'
helm-proc.el:95:20:Warning: reference to free variable `in'
helm-proc.el:98:9:Warning: reference to free variable `unless'
helm-proc.el:99:9:Warning: reference to free variable `collect'

In end of data:
helm-proc.el:208:1:Warning: the following functions are not known to be defined: loop,
    word-at-point
```

I fixed as below
- replace `cl`  macros with `cl-lib` macros(Because `helm` uses `cl-lib`)
- Import `thingatpt`(Because `word-at-point` requires this)