Improve show(); reorganize files

[kbarbary]

This PR mainly improves the results of show() to be better formatted and have more information. Here's how it looks on a dummy test file:

julia> f = FITS("tmp.fits")
File: tmp.fits
Mode: "r" (read-only)
HDUs: Num  Name    Ver  Type        
      1    image1  2    Image       
      2    image2       Image       
      3    table1       Table       
      4    table2  1    ASCIITable  

• file mode explanation shown ("read-only")
• HDU name and version now printed (version column only printed if some HDUs are versioned)

ImageHDUs:

julia> f[1]
File: tmp.fits
HDU: 1 (name="image1", ver=2)
Type: Image
Datatype: Uint32 (physical: Int32)
Datasize: (10,10)

• Name and version of HDU shown
• Data type now shown
• Physical data type shown if not the same as returned data type

TableHDUs

julia> f[3]
File: tmp.fits
HDU: 3 (name="table1")
Type: Table
Rows: 20
Columns: Name  Size  Type               TFORM    
         col3        Uint16             1I       
         col6  (2,)  Int64              2K       
         col5        ASCIIString        10A      
         col4        Bool               1L       
         col9        Array{Float64,1}*  1PD(20)  
         col2        Float64            1D       
         col7  (2,)  ASCIIString        10A5     
         col1        Int8               1B       
         col8        ASCIIString*       1PA(20)  

         (*) = variable-length column

• column TFORMs now interpreted as types, sizes and a note about whether the column is variable-length.

  Other changes in PR

• Split code for high-level interface into separate files
• Added FITSIO.libcfitsio_version() (currently not exported)
• Added name, ver keywords to write() methods for giving new extensions a name and version.
• Significant refactoring of header-keyword parsing, now uses Nullable in place of try/except. Sets the stage for specifying type of a keyword when fetching it.