Don't print coverage information to stdout

Added feature to avoid printing coverage information to stdout. This is specially useful for the use-case when we run nosetests in jenkins and we don't really care about the stdout because jenkins will parse the XML file and the stdout information just fills the log.

```
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ rm coverage.xml 
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ nosetests -s --with-xcoverage --cover-erase --xcoverage-to-stdout=0
.
----------------------------------------------------------------------
Ran 1 test in 0.015s

OK
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ head coverage.xml 
<?xml version="1.0" ?>
...
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ rm coverage.xml 
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ nosetests -s --with-xcoverage --cover-erase --xcoverage-to-stdout=1
.
Name                                                                                      Stmts   Miss  Cover   Missing
-----------------------------------------------------------------------------------------------------------------------
/usr/local/lib/python2.7/dist-packages/nose/case                                            204    163    20%   5-27, 30, 35, 44, 47-56, 61-62, 66, 71-72, 76-115, 134-138, 142, 150, 153-196, 199-217, 245-260, 264, 269, 273, 278-397
...
-----------------------------------------------------------------------------------------------------------------------
TOTAL                                                                                      2608   1940    26%   
----------------------------------------------------------------------
Ran 1 test in 0.015s

OK
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ head coverage.xml 
<?xml version="1.0" ?>
...
pablo@eulogia:~/workspace/nose-xcover-andresriancho$ 
```