Added keyword argument 'align' to table.pprint() for left/right alignment of table columns.

Added a keyword argument to the pprint function, `align`. It can take a string ('left' or 'l' for left alignment of all columns; 'right' or 'r' for right alignment of all columns; and a list of strings (following same value convention) to handle alignment of multi-column tables on a column by column basis)

An example of passing a list to `align':

    >>> from astropy.table import Table 
    >>> t = Table()
    >>> t['projects'] = ['one','two','three']
    >>> t['attributes'] = ['info1','info2','info3']
    >>> t.pprint(align=['r','l'])
    projects attributes
    -------- ----------
         one info1     
         two info2     
       three info3     

Hopefully I've done the PR correctly. I've tested these changes out for a number of different cases.