Timestamp('now', tz=xxx) result wrong
In earlier version (0.14 and earlier), Timestamp('now', tz=xxx) will return time now in xxx time zone.

In 0.15.1, it returns the time in UTC but tagged with xxx time zone.

Example: 

In version 0.12.0:

> > > import pandas
> > > pandas.version.version
> > > '0.12.0'
> > > pandas.Timestamp('now', tz='US/Central')
> > > Timestamp('2014-12-04 12:06:03-0600', tz='US/Central')
> > > pandas.Timestamp('now')
> > > Timestamp('2014-12-04 18:06:11', tz=None)

In version 0.15.1:

> > > import pandas
> > > pandas.version.version
> > > '0.15.1'
> > > pandas.Timestamp('now', tz='US/Central')
> > > Timestamp('2014-12-04 18:07:29-0600', tz='US/Central')
> > > pandas.Timestamp('now')
> > > Timestamp('2014-12-04 18:07:37')
