BUG: Timestamp cannot parse nanosecond from string

Fixes 2 problems related to ``Timestamp`` string parsing:
- ``Timestamp`` parsing results incorrect if input contains offset string (Closes #7833).

     NOTE: Also modified ``Timestamp.__repr__`` to display fixed timezone info, because this can be either ``pytz`` or ``dateutil``.

```
# Result after the fix

# If string contains offset, it will be parsed using fixed timezone offset. Following results in 2013-11-01 05:00:00 in UTC (There is some existing tests checking this behaviour).
repr(pd.Timestamp('2013-11-01 00:00:00-0500'))
# Timestamp('2013-11-01 00:00:00-0500', tz='pytz.FixedOffset(-300)')

# If tz is specified simultaneously, it should convert the timezone. 
repr(pd.Timestamp('2013-11-01 00:00:00-0500', tz='America/Chicago'))
# Timestamp('2013-11-01 00:00:00-0500', tz='America/Chicago')

repr(pd.Timestamp('2013-11-01 00:00:00-0500', tz='Asia/Tokyo'))
# Timestamp('2013-11-01 14:00:00+0900', tz='Asia/Tokyo')
```

- ``Timestamp`` loses ``nanosecond`` info when parsing from ``str``. (Closes #7878)
```
# Result after the fix

repr(pd.Timestamp('2001-01-01 00:00:00.000000005'))
# Timestamp('2001-01-01 00:00:00.000000005')

repr(pd.Timestamp('2001-01-01 00:00:00.000000005', tz='US/Eastern'))
# Timestamp('2001-01-01 00:00:00.000000005-0500', tz='US/Eastern')

repr(pd.Timestamp('2001-01-01 00:00:00.000001234+09:00'))
# Timestamp('2001-01-01 00:00:00.000001234+0900', tz='pytz.FixedOffset(540)')

repr(pd.Timestamp('2001-01-01 00:00:00.000001234+09:00', tz='Asia/Tokyo'))
# Timestamp('2001-01-01 00:00:00.000001234+0900', tz='Asia/Tokyo')

# Because non ISO 8601 format is parsed by dateutil, nanosecond will lost (no change)
repr(pd.Timestamp('01-01-01 00:00:00.000000001'))
# Timestamp('2001-01-01 00:00:00')

repr(pd.Timestamp('01-01-01 00:00:00.000000001+9:00'))
# Timestamp('2001-01-01 00:00:00+0900', tz='tzoffset(None, 32400)')
```
CC @cyber42 @ischwabacher @rockg @adamgreenhall