BUG: wrong parsing of microseconds with format arg (#4152)

Closes #4152.

`fraction` was already in microseconds, so no need to multiply it with 1000000, which caused faulty parsing of dates with microseconds.

With this commit I get:

```
pandas version: 0.12.0.dev
Value to parse: 01-Apr-2011 00:00:01.978
datetime.strptime        : 2011-04-01 00:00:01.978000
to_datetime, w/out format: 2011-04-01 00:00:01.978000
to_datetime, w/ format   : 2011-04-01 00:00:01.978000
```

for 

```
import datetime as dt
import pandas as pd
val = '01-Apr-2011 00:00:01.978'
print 'pandas version:',pd.__version__
print 'Value to parse:',val
format = '%d-%b-%Y %H:%M:%S.%f'
print 'datetime.strptime        :',dt.datetime.strptime(val, format)
print 'to_datetime, w/out format:',pd.to_datetime(val)
print 'to_datetime, w/ format   :', pd.to_datetime(val, format=format)
```