'TypeError: execv() arg 2 must contain only strings' when setting TX power

[ghost]

The following happens on Kubuntu Xenial when trying to set adapter TX power:

bacondropped@bacontop~> sudo wifite --tx 10
[sudo] password for bacondropped: 

  .;'                     `;,    
 .;'  ,;'             `;,  `;,   WiFite v2 (r87)
.;'  ,;'  ,;'     `;,  `;,  `;,  
::   ::   :   ( )   :   ::   ::  automated wireless auditor
':.  ':.  ':. /_\ ,:'  ,:'  ,:'  
 ':.  ':.    /___\    ,:'  ,:'   designed for Linux
  ':.       /_____\      ,:'     
           /       \             

 [+] TX power level set to 10

 [+] scanning for wireless devices...
 [+] enabling monitor mode on wlp7s0... done
 [+] setting Tx power to 10...
Traceback (most recent call last):
  File "/usr/local/bin/wifite", line 3525, in <module>
    engine.Start()
  File "/usr/local/bin/wifite", line 1383, in Start
    iface = self.get_iface()
  File "/usr/local/bin/wifite", line 1032, in get_iface
    return self.enable_monitor_mode(monitor)
  File "/usr/local/bin/wifite", line 898, in enable_monitor_mode
    call(['iwconfig', iface, 'txpower', self.RUN_CONFIG.TX_POWER], stdout=OUTLOG, stderr=ERRLOG)
  File "/usr/lib/python2.7/subprocess.py", line 523, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 711, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1340, in _execute_child
    raise child_exception
TypeError: execv() arg 2 must contain only strings

My PR #86 tries to address this issue by converting TX_POWER to a string.