showall produces readable tables with many columns

This PR, which should be accompanied by some clean up of our printing code, changes `showall(df)` to produce a series of Multimarkdown tables as output, each of which fits within the current TTY window. `showall(df, false)` produces a single Multimarkdown table that will not necessarily fit in the TTY window.

Some sample output:

	julia> using DataFrames

	julia> df = DataFrame(A = 1:3,
	                    B = ["Group A", "Group Alabama", "Group A"],
	                    C = ["Bigger Group A", "Bigger Group Alabama", "Bigger Group A"],
	                    D = ["Bigger Group A", "Bigger Group Alabama", "Bigger Group A"],
	                    E = ["Bigger Group A", "Bigger Group Alabama", "Bigger Group A"],
	                    F = ["Bigger Group A", "Bigger Group Alabama", "Bigger Group A"])
	DataFrame with 3 rows, 6 columns
	Columns:

	A      3 non-null values    
	B      3 non-null values    
	C      3 non-null values    
	D      3 non-null values    
	E      3 non-null values    
	F      3 non-null values    



	julia> showall(df)
	3x6 DataFrame:
	|-------|---|---------------|----------------------|----------------------|
	| Row # | A | B             | C                    | D                    |
	| 1     | 1 | Group A       | Bigger Group A       | Bigger Group A       |
	| 2     | 2 | Group Alabama | Bigger Group Alabama | Bigger Group Alabama |
	| 3     | 3 | Group A       | Bigger Group A       | Bigger Group A       |
	|-------|----------------------|----------------------|
	| Row # | E                    | F                    |
	| 1     | Bigger Group A       | Bigger Group A       |
	| 2     | Bigger Group Alabama | Bigger Group Alabama |
	| 3     | Bigger Group A       | Bigger Group A       |

	julia> showall(df, false)
	3x6 DataFrame:
	|-------|---|---------------|----------------------|----------------------|----------------------|----------------------|
	| Row # | A | B             | C                    | D                    | E                    | F                    |
	| 1     | 1 | Group A       | Bigger Group A       | Bigger Group A       | Bigger Group A       | Bigger Group A       |
	| 2     | 2 | Group Alabama | Bigger Group Alabama | Bigger Group Alabama | Bigger Group Alabama | Bigger Group Alabama |
	| 3     | 3 | Group A       | Bigger Group A       | Bigger Group A       | Bigger Group A       | Bigger Group A       |